This is one of the webpages of Libarid A. Maljian at the Department of Physics at CSLA at NJIT.
New Jersey Institute of Technology
College of Science and Liberal Arts
Department of Physics
The Earth in Space
Phys 203
Spring 2023
Fourth Examination lecture notes
Introduction to the Atmosphere
An atmosphere is a thin layer
of gas gravitationally held to a moon, a planet, or a star. The Earth’s atmosphere is roughly eighty
percent nitrogen gas, roughly twenty percent oxygen gas, and tiny amounts of
other gases. The tiny amounts of other
gases are quite important, as we will discuss shortly. Every second of every day of our lives, we
are breathing mostly nitrogen gas (roughly eighty percent) and a fair amount of
oxygen gas (roughly twenty percent).
This roughly twenty-percent abundance of oxygen gas is an enormous
fraction; other planets have nowhere nearly this much oxygen gas in their
atmospheres. Other planetary atmospheres
have only tiny amounts of oxygen gas with a large abundance of carbon dioxide
gas, as is the case with the atmospheres of planets Venus and Mars for
example. The Earth’s atmosphere has a
large fraction of oxygen gas but only a tiny amount of carbon dioxide gas. To understand why the Earth’s atmosphere is
so different from other planetary atmospheres, we must discuss the history of
the Earth’s atmosphere. When the Earth
formed roughly 4.5 billion years ago, its atmosphere was almost entirely
hydrogen gas and helium gas; this is called the
Earth’s primary atmosphere. The primary
atmosphere was almost entirely hydrogen and helium because the Earth together
with the entire Solar System was born from a nebula, an enormous cloud of gas
composed of mostly hydrogen and helium.
Indeed, most of the universe is composed of hydrogen and helium. The more massive (or heavier) an atom or
molecule, the slower it moves; the less massive (or lighter) an atom or
molecule, the faster it moves. This is
rather remarkable. Suppose all the air
in a room is at the same temperature, which means that all the air molecules in
the room have the same average energy.
Nevertheless, the oxygen molecules are moving slower on average since
they are more massive (or heavier), while the nitrogen molecules are moving
faster on average since they are less massive (or lighter), even though all of
the air is at the same temperature, which means both the nitrogen molecules and
the oxygen molecules all have the same average energy! Hydrogen is the least massive (lightest) atom
in the entire universe, and helium is the second least massive (second
lightest) atom in the entire universe.
Hydrogen and helium are so light that they move so fast that they can
escape from the Earth’s gravitational attraction. Thus, the Earth lost its primary atmosphere
because its own gravity was too weak to hold onto hydrogen and helium. This occurred with all four of the inner
planets orbiting the Sun (Mercury, Venus, Earth, and Mars). These four inner planets have weaker gravity
since they are smaller with less mass as compared with the four outer planets
orbiting the Sun (Jupiter, Saturn, Uranus, and Neptune). These four outer planets have stronger
gravity since they are larger with more mass; thus, they have retained their
primary (hydrogen and helium) atmospheres to the present day. Even the gravity of the outer planets is weak
compared with the gravity of the Sun; therefore, the Sun has certainly retained
its primary (hydrogen and helium) atmosphere to the present day. To summarize, the Sun and the four outer
planets have retained their primary (hydrogen and helium) atmospheres, but the
four inner planets have lost their primary (hydrogen and helium)
atmospheres. After the Earth lost its
primary atmosphere, this left behind a secondary atmosphere composed of an
abundance of water vapor and nitrogen gas and carbon dioxide gas. These gases came from volcanic outgassing;
since the Earth was born almost entirely molten,
volcanic eruptions everywhere on its surface ejected not just lava but gases as
well, as we discussed earlier in the course.
These gases are significantly more massive (or heavier) than hydrogen
and helium. Therefore, they did not move
fast enough to escape from the Earth’s gravitational attraction. As the Earth cooled, the water vapor condensed
into liquid water which precipitated back down onto the planet for such a long period of time that most of the surface of the Earth became
flooded, thus forming the oceans. At
this point, the Earth may be regarded as having a
normal atmosphere with an abundance of carbon dioxide gas similar to other
planets such as Venus and Mars. However,
roughly one billion years after the Earth formed
(roughly 3.5 billion years ago), something extraordinary occurred on this
planet that to our knowledge did not occur anywhere else in the entire
universe: life appeared. The first
lifeforms were probably primitive microscopic unicellular organisms, such as
bacteria and blue-green algae. Although
these lifeforms were in the ocean, they were able to extract carbon dioxide gas
from the atmosphere and replace it with oxygen gas. After roughly two billion years, these
lifeforms succeeded in extracting almost all of the carbon dioxide gas from the
atmosphere, replacing it with oxygen gas.
Thus, as of roughly 1.5 billion years ago, planet Earth attained the
atmosphere we enjoy to the present day: roughly eighty percent nitrogen gas,
roughly twenty percent oxygen gas, and tiny amounts of other gases. Again, the tiny amounts of other gases are
quite important, and we will discuss these trace gases shortly.
The pressure of the Earth’s
atmosphere is typically a maximum at mean sea level and decreases exponentially
with increasing elevation. The equation
that describes this decreasing pressure with increasing elevation is called the
law of atmospheres, but we do not need this equation to understand why this is
the case. The Earth’s gravity pulls air
downward; therefore, the air becomes thinner as we climb the atmosphere, making
the air pressure less at higher elevations.
The average air pressure at mean sea level is called one atmosphere of
pressure, equal to 101325 pascals
of pressure. One pascal
of pressure is one newton of force per square meter of area. This average air pressure of 101325 pascals is close enough to one hundred
thousand pascals that meteorologists have defined
another unit of air pressure: the bar.
One bar of air pressure is exactly one hundred thousand pascals of air pressure. Thus, the average air pressure at mean sea
level is equal to 1.01325 bars of pressure; this is also 1013.25 millibars of pressure.
When meteorologists report the air pressure on any given day, they may
report that the air pressure is several millibars
above average or several millibars below
average. A device to measure air pressure
is called a barometer.
To build a primitive barometer, we insert a long, narrow container
inverted into any liquid. The air
pressure will push downward onto the liquid, thus forcing the liquid upward
into the long, narrow inverted column.
If the air pressure is greater than average, it will push downward more
strongly onto the liquid, thus forcing the liquid further upward the inverted
column, making the column of liquid taller.
If the air pressure is less than average, it will push downward less strongly
onto the liquid, thus forcing the liquid not as far upward the inverted column,
making the column of liquid shorter.
Thus, by measuring the height of the column of liquid and performing a
calculation, we can determine the air pressure that has pushed downward onto
the liquid. Most barometers use the
element mercury as the liquid; at average air pressure at mean sea level,
liquid mercury will be pushed 760 millimeters (or 29.9
inches) up the narrow column. Thus, the
average air pressure at mean sea level is also equal to 760 millimeters of
mercury (or 29.9 inches of mercury).
When meteorologists report the air pressure on any given day, they may
report that the air pressure is several millimeters of mercury higher than
average or several millimeters of mercury lower than average. Barometers almost always
use liquid mercury because mercury is between thirteen and fourteen times more
dense than water. In other words,
mercury is between thirteen and fourteen times heavier than water; thus,
gravity pulls mercury thirteen or fourteen times more strongly than water,
making the column of mercury only 760 millimeters (or 29.9 inches) tall. If a barometer used water instead of mercury,
the column of water would be between thirteen or fourteen times taller; this
would make barometers more than ten meters (almost thirty-four feet) tall! It is not convenient to carry such a tall
device; it is much more convenient to carry a barometer that is only 760
millimeters (or 29.9 inches) tall. This
is why almost all barometers use mercury instead of water.
Meteorologists have defined
layers of the Earth’s atmosphere based on the variation of the temperature of
the Earth’s atmosphere with elevation.
The lowest layer of the atmosphere is the troposphere. With increasing elevation, the troposphere is followed by the stratosphere, then the mesosphere, and
finally the thermosphere. Beyond the
thermosphere is the exosphere, where the air smoothly transitions from the
Earth’s atmosphere to the surrounding outer space, as we will discuss
shortly. The temperature typically cools
with increasing elevation within the troposphere, the lowest layer of the
atmosphere. However, we reach a certain
elevation at which the temperature stops becoming cooler and begins instead to
become warmer with increasing elevation.
This elevation defines the end of the troposphere and the beginning of
the stratosphere. This precise elevation
is called the tropopause. We may regard the tropopause as the boundary
between the troposphere and the stratosphere, but the tropopause is more correctly defined as the end of the
troposphere. The temperature then
typically becomes warmer with increasing elevation within the
stratosphere. The reason for this
warming is a heat source within the stratosphere that we will discuss
shortly. However, we reach a certain
elevation at which the temperature stops becoming warmer and begins instead to
become cooler with increasing elevation.
This elevation defines the end of the stratosphere and the beginning of
the mesosphere. This precise elevation is called the stratopause. We may regard the stratopause
as the boundary between the stratosphere and the mesosphere, but the stratopause is more correctly defined
as the end of the stratosphere. The
temperature then typically becomes cooler with increasing elevation within the
mesosphere. However, we reach a certain
elevation at which the temperature stops becoming cooler and begins instead to
become warmer with increasing elevation.
This elevation defines the end of the mesosphere and the beginning of
the thermosphere. This precise elevation
is called the mesopause. We may regard the mesopause
as the boundary between the mesosphere and the thermosphere, but the mesopause is more correctly defined
as the end of the mesosphere. The
temperature then typically becomes warmer with increasing elevation within the
thermosphere. The reason for this
warming is a heat source within the thermosphere that we will discuss
shortly. However, we reach a certain
elevation at which the temperature stops becoming warmer and begins instead to
become cooler with increasing elevation.
This elevation defines the end of the thermosphere and the beginning of
the exosphere. This precise elevation is called the thermopause. We may regard the thermopause
as the boundary between the thermosphere and the exosphere, but the thermopause is more correctly defined
as the end of the thermosphere. The
temperature then typically becomes cooler with increasing elevation within the
exosphere, smoothly transitioning into the very cold temperatures of the
surrounding outer space. To summarize,
the temperature typically becomes cooler with increasing elevation within the
troposphere, the mesosphere, and the exosphere, while the temperature typically
becomes warmer with increasing elevation within the stratosphere and the
thermosphere.
It may seem reasonable to ask
for the precise elevation at which the Earth’s atmosphere ends and outer space
begins, but this is in fact an ill-defined question. The concentration of gases in the Earth’s
atmosphere becomes thinner and thinner with increasing elevation until we reach
a certain elevation at which the concentration of gases matches the
concentration of gases of the surrounding outer space. It is a common misconception that outer space
is a perfect vacuum, but this is false.
There is no such thing as a perfect vacuum; in fact, a perfect vacuum
would violate the laws of physics. In
actuality, the entire universe is filled with
extremely diffuse gas. Therefore, the
Earth’s atmosphere smoothly transitions into the gases of the surrounding outer
space. We may actually interpret the
Earth’s atmosphere as extending forever, filling the entire universe. The same interpretation can
be applied to all other planetary atmospheres. In other words, there is no well-defined
exopause. The tropopause is the end of
the troposphere, the stratopause is the end of the
stratosphere, the mesopause is the end of the
mesosphere, and the thermopause is the end of the
thermosphere. If there were an end of
the exosphere (which would also be the end of the entire atmosphere), that end
would be called the exopause, but there is no well-defined exopause. Nevertheless, if we insist upon a boundary
between the Earth’s atmosphere and outer space, we may arbitrarily use the
elevation of the tropopause, since the Earth’s gravity pulls most of the air in
the entire atmosphere down into the troposphere. Indeed, the vast majority of all
meteorological phenomena (commonly known as weather) occurs within the
troposphere, the lowest layer of the atmosphere. The exceptions to this are rare. For example, the jet stream is a fast-moving
current of air around the tropopause, much higher in elevation than most
meteorological phenomena (weather). So, we may arbitrarily regard the thickness of the Earth’s
atmosphere as the elevation of the tropopause as a rough estimate. The elevation of the tropopause is roughly
ten kilometers above mean sea level.
Using this as a rough estimate for the thickness of the Earth’s
atmosphere, we conclude that the atmosphere is extremely thin as compared with
the average radius of the Earth, roughly 6400 kilometers. To summarize, the Earth’s atmosphere extends
indefinitely far according to strict interpretations, but the Earth’s
atmosphere is only a few kilometers thick for all practical purposes. The Earth’s atmosphere keeps us alive in a
variety of different ways. After we
discuss all these ways the atmosphere keeps us alive, we will be humbled. In this vast universe, we are only able to
survive within a very thin layer of air surrounding a single planet: the
atmosphere of planet Earth.
The most obvious way the
Earth’s atmosphere keeps us alive is with its abundance of oxygen. Humans and all animals must inhale oxygen gas
to survive. This is because humans and
animals must react oxygen with glucose (a simple sugar) to extract the energy
they need for their survival. Humans
ingest various sugars as well as complex carbohydrates such as bread, rice,
cereal, and pasta. Our bodies digest
complex carbohydrates as well as sugars, breaking them down into glucose (a
simple sugar). There is a tremendous
amount of energy stored within the chemical bonds of the glucose molecule,
which our bodies access by reacting it with oxygen. The human body is composed of roughly one
hundred trillion cells. Within these
cells, the following reaction occurs: glucose plus oxygen yields energy plus
carbon dioxide and water as waste products.
This reaction is called cellular respiration and is more properly
written C6H12O6 + 6 O2 → energy + 6
CO2 + 6 H2O. When we inhale, the oxygen gas that goes into
our lungs is transferred to our blood; our blood then
carries the oxygen to the trillions of cells of our bodies. The oxygen enters our cells and reacts with
glucose to yield energy. The carbon
dioxide that is produced as a waste product from the
reaction is transferred back into our blood; our blood then carries the carbon
dioxide back to our lungs, and we then exhale.
Cellular respiration not only explains why humans and animals must
inhale oxygen gas; cellular respiration also explains why humans and animals
must exhale carbon dioxide gas. Plants
inhale carbon dioxide gas to use together with water as the raw materials to
synthesize glucose. Plants use the
energy of sunlight to initiate this reaction, which is why this reaction is called photosynthesis.
The photosynthesis reaction is more properly written 6 CO2 +
6 H2O
+ energy → C6H12O6
+ 6 O2. Notice that the photosynthesis
reaction is precisely the reverse of the cellular respiration reaction. Note also that oxygen is a waste product of
this photosynthesis reaction. Plants
inhale carbon dioxide gas and exhale oxygen gas, the precise reverse of humans
and animals. Therefore, the relationship
between animals (including humans) and plants is a symbiotic relationship. Animals (including humans) exhale carbon
dioxide gas, which plants then inhale.
Plants then exhale oxygen gas, which animals (including humans) then
inhale. Animals (including humans) then
exhale carbon dioxide gas, which plants then inhale, and so on and so forth.
Another way the Earth’s
atmosphere keeps us alive is by maintaining a habitable temperature for
life. Based on the Earth’s distance from
the Sun, our planet should be too cold for life to exist. The temperature of our planet should be much
colder than the freezing temperature of water; not only should all the oceans
be frozen, but the continents should be frozen over as well. However, there are tiny amounts of gases
within the Earth’s atmosphere that absorb and then emit some of the heat that
our planet radiates. These gases are called greenhouse gases.
The most important greenhouse gas is water vapor; carbon dioxide gas is
a secondary greenhouse gas. These gases
absorb some of the heat that the Earth radiates, and these gases then reradiate
this heat themselves. Although these
gases reradiate some of this heat into outer space, these gases also reradiate
some of this heat back to the Earth.
This causes the temperature of planet Earth to be significantly warmer
than it would have been otherwise, based on its distance from the Sun. In fact, the Earth is
sufficiently warmed that its average surface temperature is habitable
for life. This warming is called the greenhouse effect, since it is rather like a
greenhouse that is warm even in the wintertime.
The Earth’s atmosphere is mostly nitrogen gas and oxygen gas, but
neither of these gases can absorb or radiate heat efficiently. In other words, neither nitrogen gas nor
oxygen gas are greenhouse gases. Water
vapor and carbon dioxide gas are able to absorb and radiate heat
efficiently. The tiny amounts of water
vapor and carbon dioxide in the Earth’s atmosphere warm the planet to a
habitable temperature. This is a second
way the Earth’s atmosphere keeps us alive.
The Sun not only radiates
visible light; the Sun radiates all forms of electromagnetic radiation. The Electromagnetic Spectrum is a list of all
the different types of electromagnetic waves in order as determined by either
the frequency or the wavelength.
Starting with the lowest frequencies (which are also the longest
wavelengths), we have radio waves, microwaves, infrared, visible light (the
only type of electromagnetic wave our eyes can see), ultraviolet, X-rays, and
gamma rays at the highest frequencies (which are also the shortest
wavelengths). All of these are
electromagnetic waves. Therefore, all of
them may be regarded as different forms of light. They all propagate at the same speed of light
through the (near-perfect) vacuum of outer space for example. We now realize that whenever we use the word
light in colloquial English, we probably mean to use the term visible light,
since this is the only type of light that our eyes can actually see. The visible light band of the Electromagnetic
Spectrum is actually quite narrow.
Nevertheless, the visible light band of the Electromagnetic Spectrum can
be subdivided. In order, the
subcategories of the visible light band of the Electromagnetic Spectrum
starting at the lowest frequency (which is also the longest wavelength) are
red, orange, yellow, green, blue, indigo, and violet at the highest frequency
(which is also the shortest wavelength).
We now realize why electromagnetic waves with slightly lower frequencies
(or with slightly longer wavelengths) than visible light are
called infrared, since their frequencies (or wavelengths) are just
beyond red visible light. In other
words, infrared light is more red than red!
We also realize why electromagnetic waves with slightly higher
frequencies (or with slightly shorter wavelengths) than visible light are called ultraviolet, since their frequencies (or
wavelengths) are just beyond violet visible light. In other words, ultraviolet light is more purple
than purple! The Sun radiates all of
these electromagnetic waves. For
example, the near ultraviolet from the Sun causes suntans, and too much near
ultraviolet from the Sun causes sunburns.
The far ultraviolet has even more energy, and the Sun radiates
sufficient far ultraviolet that we should be killed from its far
ultraviolet radiation. X-rays
have even greater energy; thus, the X-rays from the Sun should kill us in a fairly short amount of time.
Something must be shielding us from the Sun’s far ultraviolet and from
the Sun’s X-rays. Our atmosphere
provides these shields. The symbol for
the oxygen atom is O. Under normal
temperatures and pressures, the oxygen atom will never remain by itself; it
will always chemically bond to another atom.
If there are no other atoms nearby, the oxygen atom will chemically bond
to another oxygen atom. Two oxygen atoms
chemically bonded to each other is called the oxygen
molecule, which is written O2. Molecular oxygen is also known as normal
oxygen, since oxygen is almost always in this state
under normal temperatures and pressures.
Roughly twenty percent of the Earth’s atmosphere is molecular (normal)
oxygen for example, and this is the form of oxygen that
plants exhale as well as the form humans and all animals must
inhale. Notice this is the form of
oxygen appearing in the cellular respiration reaction and the photosynthesis
reaction written above. Whenever we use
the simple word oxygen, we are not being clear.
Do we mean atomic oxygen O or do we mean molecular
oxygen O2? We probably mean molecular oxygen, since this
is normal oxygen. If molecular oxygen
absorbs far ultraviolet, a chemical reaction will
synthesize a very strange form of oxygen: three oxygen atoms chemically bonded
to each other. This strange form of
oxygen is written O3
and is called ozone. The synthesis of
ozone is more properly written 3 O2 + energy → 2
O3. Ozone is
toxic, since inhaling O3 causes severe
respiratory problems. This is ironic,
since ozone also keeps us alive. The molecular
oxygen in the Earth’s atmosphere absorbs the far ultraviolet from the Sun,
synthesizing ozone. Therefore, the far
ultraviolet from the Sun never reaches the surface of the Earth, since it is absorbed by molecular oxygen to synthesize ozone. In fact, there is a layer of ozone in the
stratosphere below which far ultraviolet does not penetrate. This layer is commonly
known as the ozone layer, but it is more correctly called the
ozonosphere. The ozonosphere is the heat
source within the stratosphere that is responsible for warming temperatures
with increasing elevation within that atmospheric layer. Much higher in the atmosphere within the
thermosphere, various atoms and molecules absorb X-rays from the Sun. X-rays have so much energy that absorbing
them strips electrons completely free from an atom or molecule. In other words, atoms or
molecules are ionized by X-rays.
Therefore, the X-rays from the Sun never reach the surface of the Earth,
since they are absorbed by atoms and molecules to synthesize
ionized atoms and molecules. In fact, there is a layer of ionized atoms and molecules in the
thermosphere below which X-rays do not penetrate. This layer is called
the ionosphere, and it is the heat source within the thermosphere that is
responsible for warming temperatures with increasing elevation within that
atmospheric layer.
Let us summarize all the ways
the Earth’s atmosphere keeps us alive.
Firstly, humans and animals would not have oxygen to react with glucose
to extract energy for their survival if microscopic unicellular organisms did
not remove most of the carbon dioxide gas from the atmosphere, replacing it
with molecular oxygen. Secondly, planet
Earth would be too cold for life to exist without the presence of greenhouse
gases that make the planet warm enough to be habitable for life. Thirdly, life on Earth would
be killed from the far ultraviolet from the Sun if it were not for the
ozonosphere. Fourthly, life on Earth would be killed from the X-rays from the Sun if it were not
for the ionosphere. As we discussed
earlier in the course, the atmosphere would be substantially
ionized by the Sun’s solar wind without the Earth’s magnetic field
deflecting most of these charged particles from the Sun that continually
bombard our planet. This
is a fifth way our planet keeps us alive. If only one of these were the case, we would
not be here from the lack of the other four.
If two were the case, we would not be here from the lack of the other
three. If three were the case, we would
not be here from the lack of the other two.
If four were the case, we would not be here from the lack of the
remaining one. The fact that all five of
these are the case on the same planet is truly miraculous. Again, we are humbled. In this vast universe, we are only able to
survive within a very thin layer of air surrounding a single planet: the
atmosphere of planet Earth.
The surface temperature of
the Earth causes the Earth to radiate heat from its surface. This explains why the troposphere becomes
cooler with increasing elevation; as we climb the troposphere, we are further
and further from the surface of the Earth and thus further
and further from this source of heat.
Air in the lower troposphere (near mean sea level) is
often heated by the Earth’s surface.
Since hot fluids rise, this hot air may rise to the upper troposphere
(near the tropopause). This rising air
may cool, as we will discuss shortly.
Since cool air sinks, air in the upper troposphere (near the tropopause)
may sink to the lower troposphere (near mean sea level), where it may be warmed
again thus causing it to rise again. In
summary, there is significant convection within the troposphere caused by
continually circulating air within the troposphere. This convection (circulation) of air within
the troposphere is ultimately responsible for meteorological phenomena
(commonly known as weather), as we will discuss. This explains why the vast majority of all
meteorological phenomena (weather) occurs within the troposphere, the lowest
layer of the atmosphere. This also
explains why the lowest layer of the Earth’s atmosphere is
called the troposphere, since the Greek root tropo- means turning. The ozonosphere is in the upper stratosphere,
near the stratopause.
This explains why the temperature warms as we climb the
stratosphere. After passing the
tropopause, we approach the ozonosphere, which serves as a heat source, thus
causing warming temperatures. After
passing the stratopause, we are further and further
from the ozonosphere. This explains why
the temperature cools as we climb the mesosphere. Note that cooler air resides in the lower
stratosphere while warmer air resides in the upper stratosphere. Since cool fluids do not rise, the cool air
in the lower stratosphere does not rise to the upper stratosphere. Conversely, since warm fluids do not sink,
the warm air in the upper stratosphere does not sink to the lower
stratosphere. Therefore, there is no
convection (circulation) of air within the stratosphere. The air in the stratosphere remains layered
based on temperature. This explains why
this layer of the atmosphere is called the
stratosphere, since the air is stratified or layered. The word stratify is derived from a Latin
word meaning layer. As we discussed
earlier in the course, the word stratum (a layer of sedimentary rock) derives
from the same Latin word. The air in the
mesosphere cools as we climb the mesosphere, just as air in the troposphere
cools as we climb the troposphere. This
may lead us to conclude that there is significant convection (circulation) of
air within the mesosphere just as in the troposphere, thus causing an abundance
of meteorological phenomena (weather) within the mesosphere. However, the Earth’s gravity pulls most of
the air in the entire atmosphere down into the troposphere. Although there is convection (circulation) of
air within the mesosphere, the air within this layer is too thin for this
convection (circulation) to result in an abundance of meteorological phenomena
(weather) within the mesosphere. The
Greek root meso- means middle. For example, Central America is sometimes called Mesoamerica, as in Middle America. Therefore, the word mesosphere simply means
middle sphere or middle layer. The
ionosphere is in the upper thermosphere, near the thermopause. This explains why the temperature warms as we
climb the thermosphere. After passing
the mesopause, we approach the ionosphere, which
serves as a heat source, thus causing warming temperatures. After passing the thermopause,
we are further and further from the ionosphere.
This explains why the temperature cools as we climb the exosphere. Note that cooler air resides in the lower
thermosphere while warmer air resides in the upper thermosphere. Since cool fluids do not rise, the cool air
in the lower thermosphere does not rise to the upper thermosphere. Conversely, since warm fluids do not sink,
the warm air in the upper thermosphere does not sink to the lower thermosphere. Therefore, there is no convection
(circulation) of air within the thermosphere.
The air in the thermosphere remains layered based on temperature. This is similar to the air in the
stratosphere, but note that the air in the thermosphere is much thinner than
the air in the stratosphere, since the Earth’s gravity pulls air downward. As we climb the exosphere, the air becomes
cooler and cooler, smoothly transitioning into the very cold temperatures of
the surrounding outer space. The Greek
root exo- means outside or external. For example, an exoskeleton is a skeleton
that is outside (surrounding) an organism.
Therefore, the word exosphere simply means external sphere or external
layer. The exosphere is a layer of gas
that is not strictly part of the Earth’s atmosphere but is outside
(surrounding) the Earth’s atmosphere that smoothly transitions into the gas of
the surrounding outer space.
All of us have a basic
understanding of the seasons: it is warmer in the summertime and colder in the
wintertime. It is a
common misconception that the seasons occur because of the varying
distance of planet Earth from the Sun. Supposedly when our planet Earth is closer to the Sun, it is
warmer causing summertime, and supposedly when our planet Earth is further from
the Sun, it is colder causing wintertime.
This argument seems reasonable, but it is completely wrong. The orbit of the Earth around the Sun is
almost a perfect circle, meaning that the Earth is roughly the same distance
from the Sun throughout the entire year.
Of course, the true shape of the Earth’s orbit around the Sun is an
ellipse; sometimes the Earth is closer to the Sun than average, and other times
the Earth is further from the Sun than average.
However, the eccentricity of the Earth’s elliptical orbit is so close to
zero that the orbit is almost a perfect circle.
The eccentricity of an ellipse quantifies the elongation of the
ellipse. When the eccentricity is zero,
the ellipse is a perfect circle. When
the eccentricity is close to zero, the ellipse is almost a perfect circle. The eccentricity of the Earth’s elliptical
orbit around the Sun is so close to zero that its orbit is almost a perfect
circle. When the Earth is at perihelion
(closest to the Sun), it is roughly 2.5 million kilometers (roughly 1.5 million
miles) closer to the Sun than average.
When the Earth is at aphelion (furthest from the Sun), it is roughly 2.5
million kilometers (roughly 1.5 million miles) further from the Sun than
average. These closer or further
distances may seem large, but the Earth is on average roughly 150 million
kilometers (roughly 93 million miles) from the Sun. Therefore, these closer or further distances
are less than two-percent variations from the average distance between the
Earth and the Sun, and this is not enough of a difference to cause the seasons. There is a spectacular piece of evidence that
will forever bury the misconception that the varying distance of the Earth from
the Sun causes the seasons: the Earth is closest to the Sun in wintertime and
furthest from the Sun in summertime! The
Earth is at its perihelion on roughly January 03rd
every year, but early January is in wintertime!
The Earth is at its aphelion on roughly July 03rd
every year, but early July is in summertime!
Therefore, it is absolutely not the Earth’s
varying distance from the Sun that causes the seasons. Caution: we do not argue that distance from
the Sun is completely irrelevant.
Obviously, if we were to move the Earth fifty million kilometers closer
to the Sun, of course the planet would become so hot that it would no longer be
habitable for life (all life, including all of us, would die). Obviously, if we were to move the Earth fifty
million kilometers further from the Sun, of course the planet would become so
cold that it would no longer be habitable for life (all life, including all of
us, would die). However, if we were to
move the Earth only a couple million kilometers closer to or further from the
Sun, this would not be enough to affect the Earth’s average temperature. The proof of this assertion is that this already
occurs; every year as the Earth orbits the Sun on its elliptical orbit, the
Earth moves roughly 2.5 million kilometers closer to the Sun at perihelion and
roughly 2.5 million kilometers further from the Sun at aphelion, and these
variations do not affect the average temperature of the planet. In fact, planet Earth is closest to the Sun
in wintertime and furthest from the Sun in summertime!
After discussing in
tremendous detail what does not cause the seasons, we must finally discuss what
does cause the seasons. The Earth’s
rotational axis is tilted from the vertical, the vertical being defined as
perpendicular to the plane of the Earth’s orbit around the Sun. The tilt of any planet’s rotational axis is called the obliquity of the planet. The seasons are caused
by the Earth’s obliquity, the tilt of its rotational axis. The obliquity of planet Earth is roughly 23½
degrees. As the Earth orbits the Sun,
this 23½ degrees of obliquity remains fixed to an excellent approximation. Thus, as the Earth orbits the Sun, sometimes
the Earth’s northern hemisphere will be tilted toward
the Sun, causing that hemisphere to receive more direct sunlight thus causing
warmer summertime. The
warmer summertime is also caused by daytime being longer than nighttime, as we
will discuss shortly. At the same
time the Earth’s northern hemisphere is tilted toward
the Sun, the Earth’s southern hemisphere is tilted away from the Sun, causing
that hemisphere to receive less direct sunlight thus causing colder
wintertime. The colder
wintertime is also caused by nighttime being longer than daytime, as we will
discuss shortly. Six months later
when the Earth is on the opposite side of its orbit, the Earth’s northern
hemisphere will be tilted away from the Sun, causing that hemisphere to receive
less direct sunlight and causing that hemisphere to have longer nighttime than
daytime, thus causing colder wintertime.
At the same time the Earth’s northern hemisphere is tilted away from the
Sun, the Earth’s southern hemisphere is tilted toward the Sun, causing that
hemisphere to receive more direct sunlight and causing that hemisphere to have
longer daytime than nighttime, thus causing warmer summertime. This is remarkable; the seasons are reversed in the two hemispheres at the same time! As another example, when it is spring in the
northern hemisphere, it is autumn in the southern hemisphere at the same
time. This means that the Earth is at
perihelion (closest to the Sun) during the southern hemisphere’s summertime,
and the Earth is at aphelion (furthest from the Sun) during the southern
hemisphere’s wintertime. We may be
tempted to conclude that the southern hemisphere’s summertime is especially
hot, and the southern hemisphere’s wintertime is especially cold. This is false; the opposite is true! Summers are typically hotter in the northern
hemisphere as compared with summers in the southern hemisphere, and winters are
typically colder in the northern hemisphere as compared with winters in the southern
hemisphere! In other words, both summers
and winters are more mild in the southern hemisphere
as compared with the northern hemisphere, where both summers and winters are
more severe. As we discussed earlier in
the course, this is because the continents are presently somewhat crowded together
in the northern hemisphere, making the southern hemisphere mostly covered with
ocean (water). Water has a large heat
capacity, meaning that it is difficult to change the temperature of water. Therefore, the abundance of water in the
southern hemisphere stabilizes temperatures, causing smaller temperature
variations in the southern hemisphere as compared with larger temperature
variations in the northern hemisphere.
This beautifully emphasizes that variations in the distance from the Sun
do not determine seasonal temperatures.
Again, summers are more mild (less hot) in the southern hemisphere, even
though the Earth is closest to the Sun during summertime in the southern
hemisphere, while summers are more severe (more hot) in the northern hemisphere,
even though the Earth is furthest from the Sun during summertime in the
northern hemisphere! Similarly, winters
are more mild (less cold) in the southern hemisphere, even though the Earth is
furthest from the Sun during wintertime in the southern hemisphere, while
winters are more severe (more cold) in the northern hemisphere, even though the
Earth is closest to the Sun during wintertime in the northern hemisphere! The large heat capacity of liquid water is
also responsible for moderating the temperature difference between daytime and
nighttime on our planet Earth. The
daytime side of any planet faces toward the Sun, while the nighttime side of
any planet faces away from the Sun. For
most planets, nighttime is much colder than daytime, but the nighttime side of
planet Earth is only slightly cooler than its daytime side, thanks to the
stabilizing effect of all the water that covers most of the planet. Extraordinarily, the nighttime side of planet
Earth may at times become warmer than the daytime side depending upon weather
patterns, as we will discuss. As another
example of how water stabilizes temperatures on our planet Earth, other planets
have north poles and south poles that are much colder than their equators. Although the Earth’s poles are cold and the
Earth’s equator is hot by human standards, the difference in temperature is
nevertheless moderate as compared with other planets. Without the abundance of water that covers
our planet Earth, our poles would be too cold and our equator would be too hot
to be habitable for life.
The moment when the Earth’s
northern hemisphere is tilted the most toward the Sun
is called the summer solstice. This
occurs on average June 21st every year; some years it could occur one or two
days earlier, while other years it could occur one or two days later. Since the northern hemisphere is tilted the most toward the Sun on the summer solstice,
the Sun appears to be highest in the sky, since the northern hemisphere
receives the most direct sunrays. This
is also the longest daytime and the shortest nighttime of the year in the
northern hemisphere. The precise
duration of daytime and nighttime depends upon our latitude. At the midlatitudes,
there are roughly fifteen hours of daytime and only roughly nine hours of
nighttime on the summer solstice. Note
that the sum of fifteen hours and nine hours is twenty-four hours. Six months later when the Earth is on the
other side of its orbit around the Sun, there is a moment when the Earth’s
northern hemisphere is tilted the most away from the
Sun. This moment is called the winter
solstice, occurring on average December 21st every year; some years it could
occur one or two days earlier, while other years it could occur one or two days
later. Since the northern hemisphere is tilted the most away from the Sun on the winter solstice,
the Sun appears to be lowest in the sky, since the northern hemisphere receives
the least direct sunrays. This is also
the longest nighttime and the shortest daytime of the year in the northern
hemisphere. The precise duration of
nighttime and daytime depends upon our latitude, but it will always be the
reverse of the summer solstice. At the midlatitudes for example, there are roughly fifteen hours
of nighttime and only roughly nine hours of daytime on the winter
solstice. Note again that the sum of
fifteen hours and nine hours is twenty-four hours. Halfway in between the solstices are two
other moments called the equinoxes when the Earth’s axis is not tilted toward
or away from the Sun, resulting in equal amounts of daytime and nighttime
(twelve hours each). This is why they are called equinoxes, since there are equal amounts of
daytime and nighttime. Roughly three
months after the summer solstice (roughly three months before the winter
solstice) is the autumn equinox, occurring on average September 21st every
year; some years it could occur one or two days earlier, while other years it
could occur one or two days later. Roughly three months after the winter solstice (roughly
three months before the summer solstice) is the vernal equinox (or the spring
equinox). The vernal equinox (spring
equinox) occurs on average March 21st every year; some years it could occur one
or two days earlier, while other years it could occur one or two days
later. It is a common misconception that
since every day is twenty-four hours, supposedly every day has twelve hours of
daytime and twelve hours of nighttime.
This is false for almost every day the entire year. In fact, there are only two days of the
entire year when this is the case: the equinoxes. Once we pass the vernal equinox (spring
equinox), every day for the next six months there is more daytime than
nighttime, with maximum daytime on the summer solstice. Once we pass the autumn equinox, every day
for the next six months there is more nighttime than daytime, with maximum
nighttime on the winter solstice.
The term summer solstice has
at least two different yet interrelated meanings. We may interpret the summer solstice as the
location on the Earth’s orbit where its northern hemisphere is
tilted the most toward the Sun.
We may also interpret the summer solstice as the moment in time when the
Earth’s northern hemisphere is tilted the most toward the Sun, occurring on
average June 21st every year. Obviously, the Earth is located at its
orbital summer solstice at the moment of the temporal
summer solstice. The terms winter
solstice, vernal equinox, and autumn equinox have similar interpretations. The term winter solstice has at least two
different yet interrelated meanings. We
may interpret the winter solstice as the location on the Earth’s orbit where
its northern hemisphere is tilted the most away from
the Sun. We may also interpret the
winter solstice as the moment in time when the Earth’s northern hemisphere is
tilted the most away from the Sun, occurring on average December 21st every year.
Obviously, the Earth is located at its orbital winter solstice at the moment of the temporal winter solstice. The term vernal equinox (spring equinox) has
at least two different yet interrelated meanings. We may interpret the vernal equinox (spring
equinox) as the location on the Earth’s orbit where its axis is
not tilted toward or away from the Sun as it journeys from the orbital
winter solstice toward the orbital summer solstice. We may also interpret the vernal equinox
(spring equinox) as the moment in time occurring on average March 21st every year when the Earth’s axis is not tilted toward
or away from the Sun after the temporal winter solstice but before the temporal
summer solstice. Obviously, the Earth is
located at its orbital vernal equinox at the moment of
the temporal vernal equinox. Finally,
the term autumn equinox has at least two different yet interrelated
meanings. We may interpret the autumn
equinox as the location on the Earth’s orbit where its axis is
not tilted toward or away from the Sun as it journeys from the orbital
summer solstice toward the orbital winter solstice. We may also interpret the autumn equinox as
the moment in time occurring on average September 21st
every year when the Earth’s axis is not tilted toward or away from the Sun
after the temporal summer solstice but before the temporal winter
solstice. Obviously, the Earth is
located at its orbital autumn equinox at the moment of
the temporal autumn equinox.
As we discussed earlier in
the course, the latitude of any location on planet Earth is
defined as its angle from the equator, whether north or south. The colatitude of any location on planet
Earth is defined as its angle from the north pole. Since there are ninety degrees of latitude
from the equator to the north pole, this makes the
colatitude equal to ninety degrees minus the latitude. For example, if our latitude is ten degrees
north, this means we are ten degrees of latitude north of the equator, making
us eighty degrees from the north pole; therefore, our
colatitude is eighty degrees. Indeed,
ninety minus ten equals eighty. As
another example, if our latitude is seventy degrees north, this means we are
seventy degrees of latitude north of the equator, making us twenty degrees from
the north pole; therefore, our colatitude is twenty
degrees. Indeed, ninety minus seventy
equals twenty. The only location on
planet Earth where our latitude and our colatitude equal the same number is at
forty-five degrees north latitude, since that would place us halfway between
(equidistant from) the equator and the north pole. Indeed, ninety minus forty-five equals
forty-five. The altitude of the Sun at
noon on the summer solstice equals our colatitude plus the obliquity. The altitude of the Sun at noon on the winter
solstice equals our colatitude minus the obliquity. The altitude of the Sun at noon on either
equinox equals our colatitude.
Everything we have discussed applies not just to planet Earth but also
to any other planet orbiting the Sun.
The obliquity of any planet is the angular tilt of its rotational axis
from the vertical direction that is perpendicular to its own orbital plane
around the Sun. The poles of any planet
are where its own rotational axis intersects the planet. The equator of any planet is halfway between
the two poles of the planet. Our
latitude on that planet would be our angle north or south from that planet’s
equator. The planet’s equator would be
0° latitude on that planet. The planet’s
north pole would be 90°N
latitude on that planet, and the planet’s south pole would be 90°S latitude on that planet. Our colatitude on that planet would be our
angle from that planet’s north pole, which would again
be ninety degrees minus our latitude.
The summer solstice of any planet is the moment when its northern
hemisphere is tilted the most towards the Sun. The winter solstice for any planet is the
moment when its northern hemisphere is tilted the most
away from the Sun. The equinoxes of any
planet is halfway between the solstices when its rotational axis is not tilted toward or away from the Sun. The altitude of the Sun at noon on each of
these dates would be the same equations: colatitude plus obliquity on the
summer solstice, colatitude minus obliquity on the winter solstice, and
colatitude on the equinoxes. The only
difference in this analysis for other planets are the actual dates of the
solstices and the equinoxes. For any
planet orbiting the Sun, the time from one solstice to the next solstice (which
is also the time from one equinox to the next equinox) is one-half of the
planet’s orbital period around the Sun.
The time from one solstice to the next equinox (which is also the time
from one equinox to the next solstice) is one-quarter of the planet’s orbital
period around the Sun. As an example,
consider a hypothetical planet with an obliquity of thirty degrees, and suppose
we live at fifty degrees north latitude on this hypothetical planet. Since our latitude is fifty degrees north,
our colatitude is forty degrees, since ninety minus fifty equals forty. Hence, the altitude of the Sun at noon on the
summer solstice would be seventy degrees, since the colatitude plus the
obliquity is forty plus thirty, which equals seventy. The altitude of the Sun at noon on the winter
solstice would be ten degrees, since the colatitude minus the obliquity is
forty minus thirty, which equals ten.
The altitude of the Sun at noon on either equinox would be forty
degrees, since that is our colatitude.
It is a common misconception
that the Sun is directly overhead at noon.
This misconception comes from the phrase high noon. Of course, the Sun is highest in the sky at
noon, giving this phrase some validity.
Nevertheless, the Sun is never ever directly overhead at most locations
on Earth. For example, suppose we live
at forty degrees north latitude. Our
colatitude would be fifty degrees, since ninety minus forty equals fifty. The highest the Sun would ever be at this location
is on the summer solstice, when its altitude at noon is 73½ degrees, since our
colatitude plus obliquity is fifty plus 23½, which is indeed 73½ degrees. Although 73½ degrees is a high altitude, it
is not directly overhead. Directly
overhead would be ninety degrees of altitude.
If the Sun is not directly overhead at noon on
the summer solstice, it would only be lower in the sky every other day of the
year. This shows that the Sun is never
ever directly overhead at most locations on Earth. Is there anywhere on planet Earth where the
Sun is directly overhead at noon on the summer solstice? Yes, at a latitude equal to the same number
of degrees north of the equator as the obliquity of planet Earth. To prove this, suppose we live at 23½ degrees
north latitude, then our colatitude would be 66½
degrees, since ninety minus 23½ equals 66½.
Thus, the altitude of the Sun at noon would be our colatitude 66½
degrees plus the obliquity of planet Earth 23½
degrees, but 66½ plus 23½ equals ninety degrees of altitude, directly
overhead! This location of 23½ degrees
north latitude is so important that it deserves a special name: the Tropic of
Cancer, as we discussed earlier in the course.
There is only one location on planet Earth where the Sun is directly
overhead at noon on the winter solstice: 23½ degrees south latitude. This location is so important that it
deserves a special name: the Tropic of Capricorn, as we discussed earlier in
the course. The words Cancer and
Capricorn refer to astronomical constellations of the zodiac; the reason these
lines of latitude are named for astronomical
constellations of the zodiac is beyond the scope of this course. There is only one location
on planet Earth where the Sun is directly overhead at noon on the equinoxes:
the equator at zero degrees latitude.
Thousands of years ago, primitive humans did not understand that the
Earth is a planet with a tilted rotational axis orbiting the Sun. For thousands of years, humans believed that
the motion of the Sun was responsible for the seasons. Although today we understand that it is actually the Earth that is orbiting the Sun, we
must also confess that when we look up into the sky, it does appear as if the
Sun is moving. Therefore, we should
understand the seasons from the frame of reference of the Earth, which was the
only understanding of humans for thousands of years. From the frame of reference of the Earth, the
Sun appears to be directly on top of the Tropic of Cancer on the summer solstice. For the next six months, the Sun appears to
move south, arriving on top of the equator three months later on the autumn
equinox and arriving on top of the Tropic of Capricorn three months after that
on the winter solstice. For the
following six months, the Sun appears to move north, arriving on top of the
equator three months later on the vernal equinox (spring equinox) and arriving
on top of the Tropic of Cancer three months after that on the summer
solstice. Again, it is
not the Sun that is actually moving north and south; in actuality, the
Earth is orbiting the Sun. Nevertheless,
we live on planet Earth, and so we must understand the seasons from the frame
of reference of the Earth. To summarize,
it is only possible for the Sun to appear directly overhead at noon if we live
somewhere between the Tropic of Cancer and the Tropic of Capricorn. If we live north of the Tropic of Cancer or
south of the Tropic of Capricorn, the Sun never ever appears to be directly
overhead.
The Arctic Circle is 66½
degrees north latitude, making its colatitude 23½ degrees, since ninety minus
66½ equals 23½. The altitude of the Sun
at noon at the Arctic Circle on the winter solstice would be zero degrees,
since our colatitude minus the obliquity would be 23½ minus 23½, which is
obviously zero. An altitude of zero
degrees means the Sun is on the horizon, such as during sunrise. At even more northern latitudes, the altitude
of the Sun on the winter solstice will be a negative number, which means it is
below the horizon. In other words, we
cannot see the Sun, making it nighttime even though it is noon! Before noon or after noon, the Sun will be
even further below the horizon. Thus,
the entire day is in perpetual nighttime!
The same occurs on the Antarctic Circle at 66½ degrees south latitude:
the altitude of the Sun at noon on the summer solstice is zero degrees, meaning
that it is on the horizon. At even more
southern latitudes, the altitude of the Sun on the summer solstice will be a
negative number, which means it is below the horizon. Again, we cannot see the Sun, making it
nighttime even though it is noon! Before
noon or after noon, the Sun will be even further below the horizon. Thus, the entire day is in perpetual nighttime! These extreme northern latitudes and extreme
southern latitudes are the only places on Earth where the Sun may never rise on
some days of the year and may never set on other days of the year. At the north pole, six months of continuous
nighttime occurs from the autumn equinox all the way to the vernal equinox
(spring equinox), and then six months of continuous daytime occurs from the
vernal equinox (spring equinox) all the way to the autumn equinox. The reverse occurs at the south
pole: six months of continuous nighttime occurs from the vernal equinox
(spring equinox) all the way to the autumn equinox, while six months of
continuous daytime occurs from the autumn equinox all the way to the vernal
equinox (spring equinox). Even when the
clock time is midnight, the Sun may still be in the sky causing daytime at
these extreme latitudes. This is the
origin of the phrase midnight Sun.
There are a
number of religious holidays that have their origins in the solstices and the
equinoxes. For example, Christmas Day is observed on
December 25th every year. Notice that this is shortly after the winter
solstice. Before Christmas Day became
the celebration of the birth of Jesus Christ, this was a pagan holiday
celebrating the winter solstice. Why
would pagans celebrate the day when the Sun appeared to be lowest in the sky at
noon with the most number of nighttime hours?
Primitive humans observed the Sun appear lower and lower in the sky
after the summer solstice; perhaps many primitive humans were terrified that
the Sun would continue to move downward until it disappeared below the horizon. However, by simply paying attention every
year, we observe that the Sun stops moving downward on the winter solstice, and
then begins to move upward. This was a
reason to celebrate for many ancient pagans.
This pagan celebration became the celebration of the birth of Jesus
Christ, since early Christians saw the birth of Jesus Christ as bringing more
and more light into a spiritually dark world.
In actuality, Jesus was not born on December 25th. We will never be certain of the exact day of
the birth of Jesus. Most of the details
of the vast majority of persons throughout human history, including Jesus, were never recorded.
We will also never be certain of the exact year of the birth of
Jesus. Although it is commonly believed
that the year of the birth of Jesus was anno Domini 1,
in actuality no one recorded the year that Jesus was born. Again, most of the details of the vast
majority of persons throughout human history, including Jesus, were never recorded.
The people of the world did not declare the year to be anno Domini 1
when Jesus was born. The ancient Romans
designated years using a few different numbering schemes, among which was the
number of years since the founding of the city of Rome, which was more than
seven centuries before the birth of Jesus.
Using this particular convention, the ancient Romans designated years using either of the Latin phrases “ab urbe condita” or “anno urbis
conditae,” meaning “in the year since the founding of the city.” Either Latin phrase was abbreviated AUC. This numbering
scheme continued to be used by Europeans even after
the fall of the Western Roman Empire.
More than five centuries after the birth of Jesus and roughly fifty
years after the Western Roman Empire fell, the Christian monk Dionysius Exiguus tried to determine the year that Jesus was
born. This monk declared that AUC 1278 should be reset to A.D. 525, where A.D. is the
abbreviation for the new Latin phrase “anno Domini” meaning “in the year of our
Lord,” effectively meaning the number of years since the birth of Jesus. Over the next few centuries, Europeans
retroactively changed the years of historical dates from AUC
to A.D. based on this declaration by Dionysius Exiguus. Europeans even changed the years of
historical dates before the birth of Jesus from AUC
to B.C., which is simply an abbreviation for “before Christ.” Modern scholarship has revealed that the year
that Jesus was born as determined by Dionysius Exiguus
is probably a few years in error. Modern
scholarship estimates that 6 B.C. is a more accurate estimate for the year of
the birth of Jesus. Many students are offended by this assertion. Students claim that it would be contradictory
for Jesus to have been born in the year 6 B.C., since B.C. is the abbreviation
for before Christ and no one can be born before the year of their own
birth! Again, we must remember that no
one recorded the date that Jesus was born.
We must remember that the belief that the year of the
birth of Jesus was A.D. 1 is based upon an estimate by a monk who lived roughly
five centuries after Jesus was born.
In fact, we should marvel that the estimate made by Dionysius Exiguus for an unrecorded event that occurred roughly five
centuries earlier was only a few years in error! As another example of a religious holiday
that has its origin in the solstices and the equinoxes, Easter is always the
first Sunday after the first Full Moon after the vernal equinox (spring
equinox) every year. Since Jesus was
Jewish, what is traditionally called the Last Supper
was in actuality a Passover celebration.
The Jewish calendar is a lunar calendar, and hence Jewish holy days are
determined by the cycles of the Moon.
Placing Easter on the first Sunday after the first Full Moon after the
vernal equinox (spring equinox) is an attempt to keep the date of Easter as
close as possible to the date of Passover.
A thermometer is a device to
measure temperature. The operation of a
thermometer is based on the principle of thermal
expansion and thermal contraction. Most
substances expand when they become warmer, and most substances contract when
they become colder. To build a primitive
thermometer, we take any object and measure its length at one temperature, and
we measure its different length at a different temperature. We draw marks at each of these lengths, and
we draw other marks between these two marks.
To determine the temperature, we simply read off whichever mark the end
of the object meets based on its length at that temperature. Unfortunately, most substances expand and contract
by only tiny amounts when their temperature changes. Hence, the marks are often
too close together, making differences in length difficult to
measure. However, the element mercury
expands by quite a noticeable amount as it becomes warmer and contracts by
quite a noticeable amount as it becomes colder.
Therefore, most thermometers use liquid mercury, since the marks are
then well separated and easy to read. An
actinometer is a device that measures solar
radiation. To build a primitive actinometer, we take any object and use a thermometer to
measure its initial temperature. Then,
we place the object in sunlight for a certain amount of time, perhaps one
hour. As the object absorbs solar
radiation, it becomes hotter. We use a
thermometer to measure its hotter temperature afterwards, and from the
difference in temperature between its hotter final temperature and its colder
initial temperature, we can calculate the amount of solar radiation the object
absorbed. Note that we must wrap the
object in black cloth to ensure that it absorbs all of the solar
radiation. Otherwise, the object will
only become hotter by some of the solar radiation that it absorbed, since the
object will reflect the rest of the solar radiation. It is convenient to use a bucket of water as
the object, since we know the heat capacity of water. An actinometer
would measure the greatest amount of solar radiation on the summer solstice,
and an actinometer would measure the least amount of
solar radiation on the winter solstice. However,
the summer solstice is almost never the hottest day of the year; a thermometer
measures the hottest air temperature roughly a month later, late July in the
northern hemisphere. Similarly, the
winter solstice is almost never the coldest day of the year; a thermometer
measures the coldest air temperature roughly a month later, late January in the
northern hemisphere. These delays occur
because the Earth is mostly covered with water, which
has a large heat capacity. In other
words, it is difficult to change the temperature of water. The northern hemisphere receives the most
direct sunrays on the summer solstice around June 21st, but it still takes
another month for the air to warm to maximum temperature sometime in late
July. In fact, it takes yet another
month for the ocean waters to warm to maximum temperature, late August in the
northern hemisphere. This is why most
people in the northern hemisphere take their summer vacations in August, so
that they may enjoy swimming in the ocean when it is warmest. The northern hemisphere receives the least
direct sunrays on the winter solstice around December 21st, but the oceans have
retained so much heat from the summertime that it still takes another month for
the air to cool to minimum temperature sometime in late January. In fact, it takes yet another month for the
ocean waters to cool to minimum temperature, late February in the northern
hemisphere.
Local (Small-Scale) Meteorological Dynamics
Aside from the seasonal
temperature variations we have discussed, many other variables affect the air
temperature on a daily basis, on an hourly basis, or even shorter
timescales. These variations in air
temperature cause variations in air pressure.
These variations in air pressure cause the meteorological phenomena
(commonly known as weather) that we will discuss. Air pressure is the force that the air exerts
per unit area. This pressure (force per
unit area) ultimately comes from molecular collisions. Therefore, we may interpret air pressure as
how frequently air molecules collide with each other. Warm air is at a lower pressure, while cold
air is at a higher pressure. This is
because the molecules of warm air are moving faster, enabling them to move
further from one another; hence, they collide with each other less frequently
since they are further apart from one another.
Conversely, the molecules of cold air are moving slower; they cannot
move far from one another and hence they collide with each other more frequently. Suppose all the air in a certain room is at
the same pressure, and consider a parcel of air in the middle of the room. Since the air pressure on either side of the
parcel of air is the same, it will suffer equal molecular collisions from
either side of itself. These equal
molecular collisions will balance each other, and the parcel of air will not
move. Now suppose instead that the air
on one side of the room is at a higher air pressure for whatever reason, and
suppose the air on the other side of the room is at a lower air pressure for
whatever reason. Again, consider a
parcel of air in the middle of the room.
This parcel of air will now suffer more molecular
collisions from the higher-pressure side of the room, and the parcel of
air will suffer fewer molecular collisions from the lower-pressure side of the
room. The net result is that the parcel
of air will be pushed from the higher pressure toward
the lower pressure. The parcel of air
will move, since it is pushed by a pressure
imbalance. This pressure imbalance is so
important that it deserves a name; it is called the
pressure gradient force. If the air
pressure throughout the room were the same, there would be no pressure gradient
and hence no force; the air would not move.
If there are variations in pressure, the pressure gradient force pushes
air from higher pressure toward lower pressure.
Moving air is called wind. Hence, the pressure gradient force causes
wind to blow.
A geometrical curve
connecting locations of equal air pressure is called
an isobar. There is zero pressure
gradient force along an isobar, since every point on an isobar is at the same
pressure. Therefore, the pressure
gradient force must point directly perpendicular to isobars. If the pressure gradient force did not point
directly perpendicular to isobars, then we would be able to break the force
into two components: one component directly perpendicular to the isobars and
the other component along the isobars.
However, the component along the isobars must be zero, as we just
argued. Therefore, the only component of
the pressure gradient force that can exist is the component directly
perpendicular to the isobars. The
pressure gradient force does not have two components; it only has one component
that is directly perpendicular to the isobars.
Every point on an isobar is at the same air pressure, but two different
isobars will of course be at two different pressures. Suppose as we move from one isobar to a
neighboring isobar, the pressure always drops by a definite amount, perhaps ten
millibars. If
isobars are closely spaced to each other, this means that the pressure drops by
ten millibars over a narrow distance. In other words, the pressure gradient will be
steep, thus causing strong winds. If the
isobars are widely spaced from each other, this means that the pressure drops
by ten millibars over a wide distance. In other words, the pressure gradient will be
shallow, thus causing light winds. This
is remarkable, since the pressure drop from one isobar to the neighboring
isobar is always a fixed amount: ten millibars in
these examples. Nevertheless, the ten-millibar pressure drop is steep if the isobars are closely
spaced to each other, while the ten-millibar pressure
drop is shallow if the isobars are widely spaced from each other. Again, this is remarkable: the same ten millibar pressure drop is
steep causing strong winds to blow in one case, while the same ten millibar pressure drop is shallow causing light winds to
blow in another case.
An anemometer is a device that
measures the velocity of wind (both the speed and the direction of wind). An anemometer is essentially a wind vane
together with a flag. As the wind blows,
the wind vane turns with a certain angular speed. From that angular speed, we can calculate the
speed with which the wind blows. A wind
vane shaped like a rooster is called a
weathercock. The flag reveals the
direction with which the wind blows; whichever way the flag flutters is the
direction the wind is blowing. The
Beaufort scale is a wind scale, named for the Irish oceanologist/oceanographer
Francis Beaufort. The Beaufort scale
uses numbers from zero (for no winds) to twelve (for hurricane-speed
winds). A low number on the Beaufort
would be a light wind, which is called a breeze. A middle number on the Beaufort scale would simply be called a wind. A high number on the Beaufort scale would be
a strong wind, which is called a gale.
We have already discussed
enough basic meteorology to analyze some simple weather patterns. Suppose we are at the beach in the daytime
when the Sun warms the Earth. Since
water has a large heat capacity, the ocean does not become as warm as the
continent. All of us have experienced this
while at the beach in the daytime; no matter how hot the daytime temperature,
the ocean water is not as warm as the sand.
Therefore, the air above the continent is warmer than the air above the
ocean. Thus, the air above the continent
is at a lower pressure as compared with the air above the ocean, which is at a
relatively higher pressure. Since the
pressure gradient force pushes air from high pressure toward low pressure, wind
will blow from the ocean toward the continent.
This is called the sea breeze. In meteorology, we always name wind based on
the direction it is blowing from, which is the opposite of the direction the
wind is blowing toward. For example, a wind blowing from the north
(which means it is blowing toward the south) is called
a north wind. As another example, a wind
blowing from the southwest (which means it is blowing toward the northeast) is called a southwest wind.
The sea breeze is blowing from the ocean toward the continent; hence, it
is called the sea breeze. We often feel this sea breeze while at the
beach in the daytime. The sea breeze is
a steady wind blowing from the ocean toward the continent during the
daytime. In the nighttime, the opposite
occurs. Suppose we are at the beach in
the nighttime when the Earth cools.
Since water has a large heat capacity, the ocean does not become as cold
as the continent. Perhaps some of us
have experienced this while at the beach in the nighttime; no matter how cool
the nighttime temperature, the ocean water is not as cold as the sand. Therefore, the air above the continent is
colder than the air above the ocean.
Thus, the air above the continent is at a higher pressure as compared
with the air above the ocean, which is at a relatively lower pressure. Since the pressure gradient force pushes air
from high pressure toward low pressure, wind will blow from the continent
toward the ocean. This is called the land breeze.
Again, we always name wind based on the direction it is blowing from,
which is the opposite of the direction the wind is blowing toward. The land breeze is blowing from the continent
toward the ocean; hence, it is called the land
breeze. Perhaps some of us have felt
this land breeze while at the beach in the nighttime. The land breeze is a steady wind blowing from
the continent toward the ocean during the nighttime. If we are facing the ocean, we feel the land
breeze upon our backs; if we turn our backs to the ocean, we feel the land
breeze upon our fronts. Similar to the
sea breeze and the land breeze is the valley breeze and the mountain breeze. In the daytime, the Sun warms the air. Hot air is less dense, and so hot air will be buoyed upward by the surrounding air. This is why hot air rises. Therefore, daytime winds will blow from a
valley up toward a mountain. This is called the valley breeze.
Again, we always name wind based on the direction it is blowing from,
which is the opposite of the direction the wind is blowing toward. The valley breeze is blowing from the valley
up toward the mountain; hence, it is called the valley
breeze. Perhaps some of us have felt
this valley breeze while on a mountain in the daytime. The valley breeze is a steady wind blowing
from the valley up toward the mountain during the daytime. In the nighttime, the opposite occurs. The air cools in the
nighttime. Cold air is more dense, and so cold air will descend into the
surrounding air. This is why cold air
sinks. Therefore, nighttime winds will
blow from a mountain down into a valley.
This is called the mountain breeze. Again, we always name wind based on the
direction it is blowing from, which is the opposite of the direction the wind
is blowing toward.
The mountain breeze is blowing from the mountain down into the valley;
hence, it is called the mountain breeze. Perhaps some of us have felt this mountain
breeze while in a valley in the nighttime.
The mountain breeze is a steady wind blowing from the mountain down
toward the valley during the nighttime.
To summarize, during the daytime the sea breeze blows from the ocean
toward the continent, while during the nighttime the land breeze blows from the
continent toward the ocean. During the daytime the valley breeze blows from the valley up toward
the mountain, while during the nighttime the mountain breeze blows from the
mountain down toward the valley.
Fictitious forces or pseudoforces are forces that do not actually exist; they
only seem to exist in certain frames of reference. For example, suppose we are in a stationary
car waiting at a red traffic light. When
the red traffic light turns green, we place our foot upon the car’s accelerator
pedal. As the car accelerates forward,
everyone and everything in the car feels a backward force. We actually feel ourselves pulled backward
into the backrest of our chair. Anything
hanging from the rearview mirror also swings backward. This backward force is a fictitious force or
a pseudoforce.
It does not exist; it only seems to exist within the car as the car
accelerates forward. Although everyone
and everything within the car feels this backward force, it nevertheless does
not actually exist. In actuality,
everyone and everything within the car remains stationary for a moment as the
car and its chairs accelerate forward, and hence the backrests of the chairs
accelerate forward and collide with our own backs. This is amusing: within the car we feel pulled backward into the backrests of the
chairs, but in actuality we remain stationary while the backrests of the chairs
accelerate forward into our backs!
Although we feel this backward force within the car, we nevertheless
conclude that this backward force is a fictitious force or a pseudoforce. It does
not actually exist; it only seems to exist within the car as the car
accelerates forward. As another example,
suppose we are in a moving car when we see a green traffic light turn yellow,
and so we place our foot upon the car’s brake pedal. As the car slows down, everyone and
everything in the car feels a forward force.
We actually feel ourselves pulled forward off of
the backrest of our chair. Anything
hanging from the rearview mirror also swings forward. In extreme cases, we may feel pulled forward
so strongly that our heads may collide with the windshield. This forward force is a fictitious force or a
pseudoforce.
It does not exist; it only seems to exist within the car as the car
slows down. Although everyone and
everything within the car feels this forward force, it nevertheless does not
actually exist. In actuality, everyone
and everything within the car remains in motion for a moment as the car and its
chairs and its windshield slow down, and hence the
backrests of the chairs move away from our own backs while the windshield moves
toward our heads. This is amusing:
within the car we feel pulled forward off of the
backrests of the chairs and toward the windshield, but in actuality the backrests
of the chairs move away from our backs and the windshield moves toward our
heads! Although we feel this forward
force within the car, we nevertheless conclude that this forward force is a
fictitious force or a pseudoforce. It does not actually exist; it only seems to
exist within the car as the car slows down.
As yet another example, suppose we are in a moving car when we see that
the highway ramp ahead curves to the left, and so we turn the steering wheel to
the left so that the car will remain on the highway ramp. As the car turns left, everyone and
everything in the car feels a rightward force.
We actually feel ourselves pulled rightward away from the driver’s side
of the car and toward the passenger’s side of the car. Anything hanging from the rearview mirror
also swings rightward and continues to remain suspended rightward in apparent
defiance of the Earth’s downward gravity as the car turns left! This rightward force is a fictitious force or
a pseudoforce.
It does not exist; it only seems to exist within the car
as the car turns left. Although everyone
and everything within the car feels this rightward force, it nevertheless does
not actually exist. In actuality,
everyone and everything within the car remains in forward motion as the car
turns left, and hence the driver’s side of the car turns away from us while the
passenger’s side of the car turns toward us.
This is amusing: within the car we feel pulled
rightward toward the passenger’s side of the car, but in actuality we remain in
forward motion while the passenger’s side of the car turns leftward toward
us! Although we feel this rightward
force within the car, we nevertheless conclude that this rightward force is a
fictitious force or a pseudoforce. It does not actually exist; it only seems to
exist within the car as the car turns left. As a fourth example, projectiles will appear
to suffer from deflections within a rotating frame of reference. This deflecting force is a fictitious force
or a pseudoforce.
It does not exist; it only seems to exist within the rotating frame of
reference. In actuality, the projectiles
are not deflected; the projectiles in fact continue
moving along straight paths. The frame
of reference is rotating, and the rotation of the entire frame of reference
seems to cause projectiles to deviate from straight trajectories. This particular fictitious force or pseudoforce is called the Coriolis
force, named for the French physicist Gaspard-Gustave de Coriolis who first
derived the mathematical equations describing this particular fictitious force
or pseudoforce.
The Coriolis force appears to cause rightward deflections in frames of
reference rotating counterclockwise, and the Coriolis force appears to cause
leftward deflections in frames of reference rotating clockwise. The Coriolis force appears to cause stronger
deflections if the frame of reference is rotating faster and appears to cause
weaker deflections if the frame of reference is rotating slower. The Coriolis force appears to vanish if the
frame of reference stops rotating. The
Coriolis force only appears to cause deflections; it does not cause projectiles
to speed up or slow down.
If the Earth were not
rotating, the study of the Earth’s atmosphere would be relatively simple. The pressure gradient force would simply push
wind perpendicular to isobars from high pressure toward low pressure. However, the Earth is rotating, and the
rotation of the Earth causes gross complications in the Earth’s atmosphere. The Earth rotates from west to east. When viewed from above the north
pole, the northern hemisphere is rotating counterclockwise. When viewed from above the south
pole, the southern hemisphere is rotating clockwise. Therefore, there is a
Coriolis force on planet Earth that appears to cause rightward deflections in
the northern hemisphere and appears to cause leftward deflections in the
southern hemisphere. The Coriolis
force would appear to be stronger if the Earth were rotating faster, while the
Coriolis force would appear to be weaker if the Earth were rotating
slower. The Coriolis force would appear
to vanish if the Earth were to stop rotating altogether. The Coriolis force only appears to cause
deflections; it does not cause projectiles to speed up or slow down. Finally, the Coriolis force is weak near the
equator (the Coriolis force is in fact zero at the equator), and the Coriolis
force becomes stronger and stronger as we move away from the equator toward the
poles (the Coriolis force is in fact strongest at the poles). The pressure gradient force continues to push
air perpendicular to isobars from high pressure toward low pressure, but in
addition the Coriolis force causes deflections to the right in the northern
hemisphere and deflections to the left in the southern hemisphere. Since the Coriolis force appears to cause
these deflections, wind will not blow directly perpendicular to isobars; wind
will not blow directly from high pressure toward low pressure.
A thermal is a parcel of air
in the Earth’s atmosphere. If we have a
low-pressure thermal surrounded by high pressure, the pressure gradient force
will push wind from the surrounding high-pressure air inward toward the
low-pressure thermal. At the same time,
the Coriolis force will cause deflections to the right in the northern hemisphere
and to the left in the southern hemisphere.
The net result of the pressure gradient force together with the Coriolis
force is an inward circulation of wind.
Winds will blow inward while circulating counterclockwise in the
northern hemisphere, and winds will blow inward while circulating clockwise in
the southern hemisphere. If we instead
have a high-pressure thermal surrounded by low pressure, the pressure gradient
force will push wind from the high-pressure thermal outward toward the
surrounding low-pressure air. At the
same time, the Coriolis force will cause deflections to the right in the
northern hemisphere and to the left in the southern hemisphere. The net result of the pressure gradient force
together with the Coriolis force is an outward circulation of wind. Winds will blow outward while circulating
clockwise in the northern hemisphere, and winds will blow outward while
circulating counterclockwise in the southern hemisphere. The weather pattern around a low-pressure
thermal is called a cyclone. Tornadoes and hurricanes are extreme examples
of cyclones, as we will discuss. The
weather pattern around a high-pressure thermal is called
an anticyclone. A beautiful clear day is
an extreme example of an anticyclone, as we will discuss. In summary, a cyclone is the weather pattern
around a low-pressure thermal, where winds blow inward while circulating
counterclockwise in the northern hemisphere and clockwise in the southern
hemisphere; an anticyclone is the weather pattern around a high-pressure
thermal, where winds blow outward while circulating clockwise in the northern
hemisphere and counterclockwise in the southern hemisphere.
At the center of a cyclone is
a low-pressure thermal; this low-pressure thermal has a low density and is
therefore buoyed upward by the surrounding air.
As an alternative argument, the low pressure is caused
by hot temperatures, and hot air must rise.
Either argument leads us to conclude that the low-pressure thermal at
the center of a cyclone rises. At the
center of an anticyclone is a high-pressure thermal; this high-pressure thermal
has a high density and therefore sinks downward into the surrounding air. As an alternative argument, the high pressure
is caused by cold temperatures, and cold air must
sink. Either argument leads us to
conclude that the high-pressure thermal at the center of an anticyclone
sinks. As the low-pressure thermal at
the center of a cyclone rises, the surrounding air pressure at higher
elevations decreases in accord with the law of atmospheres. Therefore, the rising low-pressure thermal
expands as its own pressure pushes the surrounding lower-pressure air. As the high-pressure thermal at the center of
an anticyclone sinks, the surrounding air pressure at lower elevations
increases in accord with the law of atmospheres. Therefore, the high-pressure thermal
contracts as the surrounding high-pressure air compresses the sinking
thermal. In summary, a cyclone is the
weather pattern around a low-pressure thermal, where winds blow inward while
circulating counterclockwise in the northern hemisphere and clockwise in the
southern hemisphere; the low-pressure thermal then rises to higher elevations
and expands. Conversely, an anticyclone
is the weather pattern around a high-pressure thermal that sinks to lower
elevations and contracts, then the winds blow outward while circulating
clockwise in the northern hemisphere and counterclockwise in the southern
hemisphere.
The most obvious way to
change the temperature of a gas is through the addition or extraction of
heat. If we add heat to a gas, we expect
it to become warmer; if we extract heat from a gas, we expect it to become
cooler. However, it is possible to
change the temperature of a gas without adding or extracting heat. If a gas expands, it must become cooler even
if no heat was extracted. This is because the expanding gas pushes the
surrounding gas. This requires work, and
work is a form of energy. The gas
extracts this energy from its own internal energy content, and so the gas becomes
cooler. Conversely
if a gas contracts, it must become warmer even if no heat was added. This is because the surrounding gas performs
work on the gas while compressing the gas.
This work is added to the internal energy
content of the gas, and so the gas becomes warmer. This is remarkable; we can actually change
the temperature of a gas without adding or extracting any heat. We can demonstrate this by performing all of
these experiments while the gas is wrapped within a
thermal insulator, which will not permit any heat to be added or
extracted. Nevertheless, the gas becomes
warmer when we compress it, and the gas becomes cooler when we expand it. We are forced to
conclude that heat and temperature are two completely different physical
concepts. Most people naively believe
that heat and temperature are essentially the same thing. After all, when we add heat to an object it often becomes warmer, and when we extract heat
from an object it often becomes cooler.
Nevertheless, we have just discussed circumstances where we can actually
change the temperature of a gas without the addition or extraction of any
heat. In fact, it is possible for a gas
to become warmer under certain circumstances when we have extracted heat from
the gas! It is also possible for a gas
to become cooler under certain circumstances when we have added heat to the
gas! These extraordinary examples
strongly persuade us that heat and temperature are two completely different
physical concepts. Therefore, we must
use different terms to describe one process where there is no temperature
change and another process where there is no heat exchanged, since heat and
temperature are two completely different physical concepts. A process where there is no temperature
change is called an isothermal process. A process where there is no heat exchanged is
called an adiabatic process. These are
two different processes. Simply because
a process is adiabatic (no heat exchanged) does not
mean that it is necessarily isothermal (no temperature change). Simply because a process is isothermal (no
temperature change) does not mean that it is necessarily adiabatic (no heat
exchanged). We just discussed two
examples of adiabatic processes that are not isothermal. A gas that expands adiabatically (no heat
exchanged) becomes cooler; since the temperature is changing, this is not an
isothermal process. A gas that contracts
adiabatically (no heat exchanged) becomes warmer; since the temperature is
changing, this is not an isothermal process either. These are two examples of processes that are
adiabatic yet not isothermal, meaning the temperature changes even though no
heat was exchanged.
There are other examples of processes that are isothermal yet not
adiabatic, meaning heat was exchanged even though the
temperature did not change. Obviously,
there are processes that are neither adiabatic nor isothermal. Also note that a
process where there is no pressure change is called an isobaric process.
Air is a poor conductor of
heat. A spectacular illustration of the
poor conduction of heat through air is the moderate heat we feel from the
intense temperature of charcoal during a barbecue. When the charcoal begins glowing red, its
temperature is a couple of thousand degrees!
Yet, we can place our hands within just a few inches of the charcoal;
although we feel moderate heat, our hands are not in danger from the extreme
temperature of the charcoal. How can our
hands be within a few inches of something with a couple of thousand degrees of
temperature and yet not be in any danger?
We conclude that the air between our hands and the hot charcoal is a
poor conductor of heat. Since air is
such a poor conductor of heat, we always assume thermals in the Earth’s
atmosphere neither gain heat from their surroundings nor lose heat to their
surroundings. That is, we always assume
thermals in the Earth’s atmosphere do not exchange heat with their
surroundings. This is
called the adiabatic approximation, and it is an excellent approximation
for most meteorological processes. As we
discussed, the air at the center of a cyclone rises and expands. We conclude from the adiabatic approximation
that the rising thermal expands adiabatically.
If the thermal expands adiabatically, then it must cool. As we discussed, the air at the center of an
anticyclone sinks and contracts. We
conclude from the adiabatic approximation that the sinking thermal contracts
adiabatically. If the thermal contracts
adiabatically, then it must warm. In
summary, a cyclone is the weather pattern around a low-pressure thermal where
winds blow inward while circulating counterclockwise in the northern hemisphere
and clockwise in the southern hemisphere; the low-pressure thermal then rises,
expands adiabatically, and cools.
Conversely, an anticyclone is the weather pattern around a high-pressure
thermal that sinks, contracts adiabatically, and warms, then the winds blow
outward while circulating clockwise in the northern hemisphere and
counterclockwise in the southern hemisphere.
The low-pressure air at the
center of a cyclone rises, expands adiabatically, and cools, but cold air is at
a high pressure. We conclude that low
air pressure regions at lower elevations in the troposphere transition to high
air pressure regions at higher elevations in the troposphere. In brief, low air pressure near mean sea
level becomes high air pressure aloft.
The word aloft simply means up toward the sky. The loft of a house is the highest room in
the house, often the attic, the choir loft of a church is high above the pews,
and lofty dreams are high goals. If
winds blow inward toward the low air pressure near mean sea level, these winds
must eventually blow outward from the high air pressure aloft. This circulation of air is a necessary
feature of convection, as we discussed earlier in the course. Cyclones may initially form from low air
pressure at lower elevations in the troposphere (near mean sea level) or from
high air pressure at higher elevations in the troposphere (aloft), but
generally cyclones initially form from both occurring in conjunction with each
other, from the appropriate vertical motion of air throughout the
troposphere. Similarly, we conclude that
high air pressure regions at lower elevations in the troposphere transition to
low air pressure regions at higher elevations in the troposphere. In brief, high air pressure near mean sea
level becomes low air pressure aloft. If
winds blow outward from the high air pressure near mean sea level, these winds
must blow inward toward the low air pressure aloft. Again, this circulation of air is a necessary
feature of convection, as we discussed earlier in the course. Anticyclones may initially form from high air
pressure at lower elevations in the troposphere (near mean sea level) or from
low air pressure at higher elevations in the troposphere (aloft), but generally
anticyclones initially form from both occurring in conjunction with each other,
from the appropriate vertical motion of air throughout troposphere.
Relative humidity is a
concept that everyone believes that they understand, but in actuality
almost no one correctly understands this concept of relative humidity. Most people believe that the relative
humidity of air is the amount of moisture in the air. This is such a gross simplification of the
correct definition of relative humidity that it is actually an incorrect
understanding. Firstly, air is only able
to hold a maximum amount of moisture. We
may demonstrate this with the following experiment. We place a lid upon a cup of water; a simple
piece of paper will serve as a satisfactory lid. Liquid water continuously evaporates into
water vapor, but the lid will confine the water vapor in the trapped air
between the lid and the liquid water.
When this confined air holds the maximum amount of water vapor that it
is able to hold, some of the water vapor must condense back into liquid water
so that additional liquid water may evaporate into water vapor. Some of this water will condense back into
the liquid within the cup, but some of this water will condense into drops of
liquid water on the sides of the cup and even underneath the lid. When the air holds the maximum amount of
moisture that it is able to hold, the air is said to be
saturated. To saturate anything
means to fill it to capacity; the air is saturated
when it holds the maximum moisture that it is able to hold. The saturation amount of the air is itself a
function of temperature. Warm air has a
greater saturation amount since warm air is able to hold a greater quantity of
moisture, while cold air has a lesser saturation amount since cold air is not
able to hold as much moisture as warm air.
The strict definition of the relative humidity of air is the amount of
moisture in the air as a fraction of the saturation amount at the given
temperature. Let us devote some time to carefully understand this definition. Firstly, the relative humidity of air is directly related to the amount of moisture in the
air. Adding moisture to air increases
the relative humidity, while subtracting moisture from air decreases the
relative humidity. However, it is
possible to change the relative humidity of air without adding or subtracting
water. By simply changing the
temperature of the air, we change the saturation amount of the air and thus we
change the fraction of the moisture to the new saturation amount. If the air becomes warmer, the saturation
amount is greater, making the amount of moisture that is actually within the
air a smaller fraction of that greater saturation amount. In other words, warming air decreases its
relative humidity. If the air becomes
colder, the saturation amount is lesser, making the
amount of moisture that is actually within the air a greater fraction of that
lesser saturation amount. In other
words, cooling air increases its relative humidity. An analogy would be helpful to understand
these processes. Imagine a large bucket
and a small cup. A large bucket is able
to hold a large amount of water, while a small cup is only able to hold a small
amount of water. The large bucket is
analogous to hot air, since hot air has a large saturation amount, meaning it
is able to hold more moisture. The small
cup is analogous to cold air, since cold air has a small saturation amount,
meaning it is not able to hold as much moisture as hot air. Now suppose the small cup is mostly
full. If we pour this water into a large
empty bucket, the large bucket will be mostly empty. If we take a large bucket that is mostly
empty and pour this water into a small cup, the small cup will be mostly full. This is remarkable: it is the same amount of
water in the small cup and the large bucket.
Nevertheless, this same amount of water makes the small cup mostly full
and makes the large bucket mostly empty.
We always keep in mind that the small cup is analogous to cold air and
the large bucket is analogous to hot air.
If we take cold air and warm it, this decreases the relative humidity,
since this is analogous to taking a small cup that is more
full and pouring its water into a large bucket that will now be more
empty. If we take warm air and cool it,
this increases the relative humidity, since this is analogous to taking a large
bucket that is more empty and pouring its water into a
small cup that will now be more full.
This is remarkable: we have not changed the amount of moisture in the
thermal. We are changing its relative
humidity without adding or extracting water; we are changing its relative
humidity by changing its temperature. We
now realize that regarding relative humidity as simply the amount of moisture
in the air is a grossly incorrect understanding of relative humidity. As another remarkable example, consider two
thermals both at fifty percent relative humidity. Does this mean they hold the same amount of
moisture? Isn’t
fifty percent equal to one-half?
Therefore, are not both thermals holding half of their respective
saturation amounts? This is certainly
true, but two thermals will almost always have two
different temperatures. The hotter
thermal has a greater saturation amount, while the colder thermal has a lesser
saturation amount. Half of a greater
amount is greater, and half of a lesser amount is lesser. Thus, the warmer thermal actually holds more
moisture and the cooler thermal actually holds less moisture, even though both
thermals have the same relative humidity!
This is analogous to a large bucket and a small cup that are both half
full. The large bucket holds more water
and the small cup holds less water, even though both are half
full! We always keep in mind that
the small cup is analogous to cold air and the large bucket is analogous to hot
air. If a large bucket and a small cup
are both half full and yet the large bucket holds more water and the small cup
holds less water, we conclude that two thermals both at fifty percent relative
humidity hold different quantities of moisture.
The warmer thermal holds more moisture (analogous to the large bucket),
while the cooler thermal holds less moisture (analogous to the small cup). As an extreme example, consider two thermals:
one at ninety percent relative humidity and the other at ten percent relative
humidity. Which thermal holds more
moisture, and which thermal holds less moisture? We are tempted to conclude that surely it
must be the ninety-percent humid thermal that holds more moisture, and we are
tempted to conclude that surely it must be the ten-percent
humid thermal that holds less moisture.
In actuality, we cannot draw any conclusions about the moistures of the
two thermals without knowing their temperatures. Again, we imagine a large bucket and a small
cup. Suppose the large bucket is only
ten-percent full, while the small cup is ninety-percent full. Nevertheless, suppose the large bucket is so
large that it still holds more water at ten-percent capacity than the small cup
at ninety-percent capacity. We always
keep in mind that the small cup is analogous to cold air and the large bucket
is analogous to hot air. Suppose we have
two thermals: one at ninety percent relative humidity and the other at ten
percent relative humidity. Now suppose
that the ten-percent-humid thermal is so warm that its saturation amount is so
large that ten percent of that large saturation is nevertheless more moisture
than the ninety-percent-humid cold thermal.
Therefore, it is possible for a ten-percent-humid thermal to hold more
moisture than a ninety-percent-humid thermal if the ten-percent-humid thermal
is sufficiently warm and if the ninety-percent-humid thermal is sufficiently
cold.
Since warming air decreases
the relative humidity, the least humid time of the day is typically in the late
afternoon just before sunset, since the Sun has spent the entire daytime
warming the air. Since cooling air
increases the relative humidity, the most humid time of the day is typically in
the very early morning hours just before sunrise, since the air has spent the
entire nighttime cooling in the darkness.
Just before sunrise, the air may have cooled sufficiently for the
relative humidity to increase to one hundred percent. That is, the air has become
saturated with water vapor. At
one hundred percent relative humidity (saturation), water vapor must condense
into liquid water so that additional liquid water may evaporate. The temperature to which we must cool air
until it becomes saturated is called the dew point,
since the water vapor that condenses into liquid water is called dew. In the early morning, we may see the leaves
of trees and the surface of our car covered with water as if it had rained
overnight. In actuality, it became
sufficiently cold overnight that the dew point was achieved. The air became saturated, and water vapor
began condensing into liquid water. Even
in the summertime, the nighttime air may become sufficiently cold that the dew
point is achieved, thus forming dew.
Condensation is the changing
of state from water vapor to liquid water.
The condensing water must liberate heat to its surroundings to condense;
this liberated heat warms the surroundings.
Therefore, condensation is a warming process. Evaporation is the changing of state from
liquid water to water vapor. The evaporating water must extract heat from
its surroundings to evaporate; this extracted heat cools the surroundings. Therefore, evaporation is a cooling process. This is why we feel chilly immediately after
taking a shower. Our bodies are covered with water that evaporates; the evaporating
water extracts the heat needed for that evaporation from our bodies, thus
cooling our bodies. Drying our bodies
with a towel removes water that would have evaporated; hence, we feel less
chilly whenever we dry ourselves with a towel.
This is also why humans and some animals
perspire (sweat). The act of perspiring
(sweating) covers our bodies with water that evaporates; the evaporating water
extracts the heat needed for that evaporation from our bodies, thus cooling our
bodies. Suppose the surrounding air is
very humid, perhaps close to saturation.
Some of the water vapor in the air must condense to liquid water so that
additional liquid water may evaporate. Whereas
perspiration (sweat) on our skin may evaporate which cools our bodies, some
water vapor in the surrounding air condenses to liquid water onto our skin,
adding heat back to our bodies, since condensation is a warming process. Therefore, our bodies cannot cool
effectively, and we feel uncomfortable.
As a result, humid air feels warmer than its actual temperature. We can convert this
uncomfortable feeling into an effective air temperature that is warmer than the
actual air temperature. This effective
air temperature is called the heat stress index (or
the heat index for short). For example,
a meteorologist may report in the summertime that the actual temperature today
will be ninety degrees fahrenheit,
but it will feel like ninety-five degrees fahrenheit. The ninety degrees fahrenheit is the true air temperature, while the
ninety-five degrees fahrenheit is the heat stress
index (or simply the heat index). In the
wintertime, wind makes the air feel colder than its actual temperature. We can convert this
uncomfortable feeling into another effective air temperature that is colder
than the actual air temperature. This
effective air temperature is called the windchill. For
example, a meteorologist may report in the wintertime that the actual temperature
today will be thirty-five degrees fahrenheit,
but it will feel like twenty-five degrees fahrenheit. The thirty-five degrees fahrenheit is the true air temperature, while the
twenty-five degrees fahrenheit is the windchill. We may
use all of these principles to construct a hygrometer, a device that measures
the relative humidity of the air. A
hygrometer is simply two thermometers.
One thermometer is wrapped in a wet cloth; this
is called the wet-bulb thermometer. The
other thermometer that is not wrapped in a wet cloth
is called the dry-bulb thermometer.
Water will evaporate from the wet-bulb thermometer. Since evaporation requires heat, the
evaporating water will extract heat from the wet-bulb thermometer, giving it a
colder temperature than the dry-bulb thermometer. Again, evaporation is a cooling process. From the difference in temperature between
the wet-bulb thermometer and the dry-bulb thermometer, we can calculate the
relative humidity of the air.
We now apply everything we
have discussed about relative humidity to cyclones and anticyclones. The low-pressure, low-density thermal at the
center of a cyclone rises, expands adiabatically, and cools. Since it cools, its relative humidity
increases. The high-pressure,
high-density thermal at the center of an anticyclone sinks,
contracts adiabatically, and warms.
Since it warms, its relative humidity decreases. In summary, the thermal at the center of a
cyclone becomes more humid as it rises, while the thermal at the center of an
anticyclone becomes less humid (or more dry) as it sinks. If the thermal at the center of a cyclone
becomes more humid as it rises, the dew point could be
achieved, causing water vapor to condense into liquid water. However, even if the dew point is achieved, water vapor cannot condense into liquid water
in midair. The liquid water requires a
surface upon which to condense, such as the surface of the leaves of trees or
the surface of our car. Fortunately, the
atmosphere is not just air; there are tiny pieces of dust and silt and salt in
the atmosphere. When the dew point is achieved, the water vapor can condense into liquid water
around these tiny pieces of dust and silt and salt, forming a microscopic drop
of water around each tiny piece of dust or silt or salt. For this reason, this tiny piece of dust or
silt or salt is called a condensation nucleus, since
it is at the center of the microscopic drop of water. The center of anything is
called its nucleus. For example,
the center of a biological cell is called the cellular
nucleus, the center of an atom is called the atomic nucleus, and the center of
an entire galaxy is called the galactic nucleus. If the dew point is achieved causing water
vapor to condense into microscopic drops of water around these condensation
nuclei, the thermal becomes opaque.
Ordinarily, air is transparent, as we know from our daily
experience. Every second of every day of
our lives, we effortlessly see through the air around us, since air is
ordinarily transparent. However, liquid
water is opaque. Actually, a small
quantity of liquid water is transparent.
We can see through a glass of water for example. However, larger and larger quantities of
liquid water become less and less transparent and more and more opaque. It is rather difficult seeing through a fish
tank for example, and it is hopeless trying to see through the ocean to the
seafloor. Therefore, when the dew point
is achieved causing water vapor to condense into liquid water, the thermal does
indeed become opaque. We can no longer
see through the air; the thermal has turned from invisible to visible. This is so remarkable that the thermal
deserves a special name. A thermal that
has achieved the dew point causing its water vapor to condense into liquid
water and thus the thermal has turned from transparent to opaque (from
invisible to visible) is called a cloud.
We conclude that clouds form when thermals rise, expand adiabatically,
cool, and become more humid until the dew point is achieved. This dew point is a specific temperature. Therefore, thermals must be
lifted to a specific elevation to cool to the dew point. This elevation is called
the lifting condensation level (or the condensation level for short). We can almost always
see the lifting condensation level (or simply the condensation level) with our
own eyes, since clouds often have flat bottoms.
This flat bottom is the lifting condensation level (or simply the
condensation level). Below this
elevation, the dew point has not been achieved, and
the thermals are still transparent (invisible).
Above this elevation, the dew point has been achieved,
and the thermals are opaque (visible) clouds.
We can categorize clouds into
three broad types: cumulus clouds, cirrus clouds, and stratus clouds. Cumulus clouds have the appearance of
cauliflower or puffs of cotton. Cirrus
clouds have the appearance of individual wisps or feathers. Finally, if there are so many clouds in the
sky that they all blend together to form one giant layer of cloud covering the
entire sky, this is a stratus cloud. The
word stratus is derived from a Latin word meaning
layer. As we discussed, the word
stratify (meaning layered) is derived from the same Latin word. The word stratum (a layer of sedimentary
rock) also derives from the same Latin word, as we discussed earlier in the
course. Note that there are other cloud
types in addition to these three. For
example, many cirrus clouds that seem to almost blend together into one giant
layer of cloud covering the entire sky is called a cirrostratus cloud, meaning
intermediate between cirrus clouds and stratus clouds. As another example, many cumulus clouds that
seem to almost blend together into one giant layer of cloud covering the entire
sky is called a stratocumulus cloud, meaning intermediate between cumulus
clouds and stratus clouds. If it is
precipitating (raining or snowing) out of a cumulus cloud, then it is called a
cumulonimbus cloud. If it is
precipitating (raining or snowing) out of a stratus cloud, then it is called a
nimbostratus cloud. If the air becomes
sufficiently cold that it achieves the dew point without having to be pushed up to higher elevations, a cloud will form at or
near the ground. This type of cloud is called fog.
We have already discussed
enough meteorology to somewhat reliably predict
weather over a timescale of a few hours using only a barometer. The rising or the falling of the air pressure
as indicated by the barometer is called the barometric
tendency. If the barometric tendency is
falling, then low-pressure, low-density thermals must be rising, expanding
adiabatically, cooling, and becoming more humid. The relative humidity may increase
sufficiently so that the dew point will be achieved,
forming clouds and perhaps even precipitation (rain or snow). Conversely, if the barometric tendency is
rising, then high-pressure, high-density thermals must
be sinking, contracting adiabatically, warming, and becoming less humid. The relative humidity may decrease
sufficiently so that liquid water will evaporate back into water vapor. In other words, thermals will turn from
opaque (visible) clouds to transparent (invisible) air; we will have a clear
day. In summary, a falling barometric
tendency is an indication of what is commonly considered to
be bad weather, while a rising barometric tendency is an indication of
what is commonly considered to be good weather.
The rate at which a rising
thermal cools before it becomes a cloud is called the
dry adiabatic rate of cooling (or the dry adiabatic rate for short). The rate at which a rising thermal cools
after it becomes a cloud is called the wet adiabatic
rate of cooling (or the wet adiabatic rate for short). A thermal becomes a cloud when water vapor
condenses into liquid water. This
liberates heat, thus making the thermal warmer.
Again, condensation is a warming process. Therefore, the wet adiabatic rate of cooling
is always more shallow than the dry adiabatic rate of cooling. Stated the other way around, the dry
adiabatic rate of cooling is always more steep than the wet adiabatic rate of
cooling. The rate at which the
surrounding atmosphere cools with rising elevation is called
the environmental lapse rate of cooling (or the environmental lapse rate for
short). Suppose the environmental lapse
rate is more shallow than the wet adiabatic rate which
itself must be more shallow than the dry adiabatic rate. In other words, both adiabatic rates are more steep than the environmental lapse rate. In this case, either before or after a
thermal becomes a cloud, its rate of cooling is very steep. The rate of cooling of the thermal may be
sufficiently steep that the thermal becomes so cold and so dense that it is forced to sink to lower elevations. This is called absolute stability, and what
is commonly considered to be bad weather such as
clouds or precipitation (rain or snow) will be less likely. Conversely, suppose the environmental lapse
rate is steeper than the dry adiabatic rate which itself must be steeper than
the wet adiabatic rate. In other words,
both adiabatic rates are more shallow than the
environmental lapse rate. In this case,
either before or after a thermal becomes a cloud, its rate of cooling is
shallow. Thermals will probably not cool
sufficiently to become dense enough to sink to lower elevations. In other words, thermals are more likely to
rise to higher elevations. This is
called absolute instability, and what is commonly considered to
be bad weather such as clouds and perhaps even precipitation (rain or
snow) will be more likely. It is also
possible for the environmental lapse rate to be steeper than the wet adiabatic
rate but more shallow than the dry adiabatic
rate. In this case, a thermal before it
becomes a cloud may cool to attain sufficiently high density that it may sink,
resulting in what is commonly considered to be good weather. However, if the thermal reaches the lifting
condensation level and becomes a cloud, its rate of cooling slows. Hence, the thermal will continue to rise,
resulting in what is commonly considered to be bad
weather. This is
called conditional instability, and either good weather or bad weather
may result under these circumstances.
Our discussion leads us to
conclude that weather is strongly determined by lifting, the rising of
thermals. There are
three mechanisms that could cause lifting: orographic lifting, convergence
lifting, and frontal wedging.
Orographic lifting is caused by mountains
pushing air aloft. This term is derived from the Greek root oro-
for mountain, as we discussed earlier in the course. When winds encounter a mountain, much of the
air blows up over the mountain. As the
air rises, it expands adiabatically, cools, and becomes more humid. If the dew point is
achieved, clouds form, and precipitation may occur. Therefore, we expect a humid climate on the
windward side of a mountain range. The
windward side of anything is the side that faces the wind. The opposite of the windward side of anything
is its leeward side, which faces away from the wind. The term leeward side is
sometimes shortened to the word lee, such as the lee of a building or
the lee of a rock being the side that faces away from the wind. On the leeward side of a mountain range, air
sinks, contracts adiabatically, warms, and becomes less humid. Therefore, we expect an arid (dry) climate on
the leeward side of a mountain range.
Actually, we expect an arid (dry) climate for an additional reason: any
moisture that was in the air probably precipitated out of the air on the
windward side of the mountain range.
With moisture subtracted and in addition warming temperatures from
sinking air, we expect an extremely arid (dry) climate on the leeward side of
mountain ranges. These are called rainshadow
deserts. For example, the contiguous
United States is at the midlatitudes, and the
prevailing winds at the midlatitudes blow from the
west, as we will discuss. Therefore, the
west side of the Rocky Mountains is its windward side, while the east side of
the Rocky Mountains is its leeward side.
The leeward side (the east side) of the Rocky Mountains is the Great
Plains of the United States, a rainshadow
desert. Although there is agriculture in
the Great Plains, the soil is not as productive as the farmland of the midwestern United States, which is further east of the
Great Plains. Convergence lifting is caused by crowded winds pushing air aloft. Consider an island or a peninsula surrounded
on many sides by water. Every day, sea
breezes will blow from the surrounding waters toward the island or peninsula,
as we discussed. These breezes become
crowded and thus push each other upward.
As the air rises, it expands adiabatically, cools, and becomes more
humid. If the dew point is achieved, clouds form, and precipitation may occur. Therefore, we expect islands and peninsulas
to have humid climates. Actually, we
expect the climate to be extremely humid, since the winds originally came from
the surrounding waters, where evaporation added significant moisture to the sea
breezes. With moisture added and in
addition cooling temperatures from rising air, we expect extremely humid
climates on islands and peninsulas. For
example, Florida is a peninsula in the southeastern United States. Every day, a sea breeze blows from the Gulf
of Mexico from the west towards Florida.
Every day, a sea breeze blows from the Atlantic Ocean from the east
towards Florida. Every day, a sea breeze
blows from the Caribbean Sea from the south towards Florida. These sea breezes were already humid, since
they came from bodies of water where evaporation added moisture to the
winds. In addition, these sea breezes
become crowded over Florida and thus push each other upward. The air rises, expands adiabatically, cools,
and becomes even more humid. The dew
point is achieved, clouds form, and rain occurs. This explains why Florida has an extremely
humid climate. In fact, the entire
peninsula is infested with amphibians and reptiles as a
result of this extreme humidity.
Frontal wedging is caused by one air mass
pushing another air mass aloft. This is
the most important type of lifting that determines weather patterns, as we will
discuss.
The lightest type of liquid
precipitation is called mist. Heavier than mist is drizzle, and the
heaviest liquid precipitation is called rain.
The lightest freezing precipitation is called
snow. Heavier than snow is freezing
drizzle. Heavier than freezing drizzle
is called sleet. Even heavier than sleet
is called graupel, and hail is the heaviest freezing
precipitation. Hail is quite dangerous;
many people have been killed from falling hail. Snow is very light because it is composed of
individual snowflakes, which are themselves composed of mostly air. Since clouds form
aloft (at higher elevations) in the troposphere where the air temperature is
colder, precipitation almost always begins in the frozen state, such as snow or
sleet. On its way down to lower
elevations, the precipitation warms and melts into liquid precipitation such as
rain. This is usually the case even in
the summertime; warm rain in the summertime most likely began as snow or sleet
from clouds at higher elevations in the troposphere that melted into rain on
its way down toward lower elevations in the troposphere. We can personally experience this extreme
temperature difference between the lower troposphere and the upper troposphere
with a sufficiently tall mountain. As we
climb the mountain, the air temperature becomes colder and colder until we
reach the summit of the mountain, where it may be so cold that it is snowing. This is usually the case even in the
summertime. The air temperature at the
bottom of the mountain may be quite hot in the summertime. Nevertheless, the air temperature becomes
colder and colder as we climb the mountain until (if the mountain is
sufficiently tall) the air temperature is so cold that it is snowing at the
summit of the mountain, even in summertime when the base of the mountain is
still quite hot!
Global (Large-Scale) Meteorological Dynamics
The Coriolis force caused by
the Earth’s rotation causes the global circulation of air in the atmosphere to
be complex. In order to emphasize the
complications caused by the rotation of the Earth, let us first suppose that
the Earth were not rotating. In this
case, the global circulation of air in the atmosphere would be simple. Since the equator is hot throughout the
entire year, the air at the equator is at low pressure. Since the poles are cold throughout the
entire year, the air at the poles is at high pressure. The pressure gradient force would then push
air from high pressure at the poles toward low pressure at
the equator. The result is that
winds would blow from the north in the northern hemisphere and from the south
in the southern hemisphere. Since we
always name wind based on the direction it is blowing from, the winds in the
northern hemisphere would be a north wind, while winds in the southern
hemisphere would be a south wind. These are known as the prevailing winds. Caution: wind does not always blow in the
directions of these prevailing winds; variations in pressure may cause winds to
blow in various different directions.
The prevailing winds are the directions in which the wind generally or
usually blows, not the directions in which the wind always blows. If the Earth were not rotating, winds in the
northern hemisphere would generally or usually be a north wind from the north
pole toward the equator, and winds in the southern hemisphere would generally
or usually be a south wind from the south pole toward the equator. Once at the equator, the low-pressure, low-density
air would rise, becoming high pressure aloft.
The high pressure at the poles is low pressure aloft. Again, the pressure gradient force pushes air
from high pressure toward low pressure.
Hence, the pressure gradient force would push the risen air at the
equator toward the poles, where the air would sink until the pressure gradient
force pushes the air back toward the equator.
This overall motion is called a circulation
cell. Notice there would be only one
circulation cell in each hemisphere if the Earth were not rotating. This discussion completely summarizes the
global circulation of air in the atmosphere if the Earth were not rotating.
Of course, the Earth is
rotating, causing a Coriolis force and hence tremendous complications to the
simplistic model we have just presented.
The low-pressure, low-density air still rises at the equator, and the
pressure gradient force still pushes this risen air toward the poles. However, by the time the air reaches roughly
thirty degrees latitude in each hemisphere, the air has cooled sufficiently to
sink. The pressure gradient force then
pushes this air back toward the equator, completing the tropical circulation
cells. However, the Coriolis force
causes rightward deflections in the northern hemisphere and leftward
deflections in the southern hemisphere.
The net result of the pressure gradient force together with the Coriolis
force is that the prevailing winds (near mean sea level) from roughly 30°N latitude to 0° latitude (the equator) blow from the
northeast; these are called the northeast trade winds, since we always name
wind based on the direction it is blowing from.
The prevailing winds (near mean sea level) from roughly 30°S latitude to 0° latitude (the equator) blow from the
southeast; these are called the southeast trade winds, since we always name
wind based on the direction it is blowing from.
The term trade wind is used since these winds
pushed sailing ships across the Atlantic Ocean from Europe and Africa toward
North America and South America. Thus,
these winds facilitated trade between the Old World and the New World. The high-pressure, high-density air still
sinks at the poles, and the pressure gradient force still pushes this air
toward the equator. However, by the time
the air reaches roughly sixty degrees latitude in each hemisphere, the air has
warmed sufficiently to rise. The
pressure gradient force still pushes this risen air toward the poles where it
sinks, completing the polar circulation cells.
However, the Coriolis force causes rightward deflections in the northern
hemisphere and leftward deflections in the southern hemisphere. The net result of the pressure gradient force
together with the Coriolis force is that the prevailing winds (near mean sea
level) from 90°N latitude (the north pole) to roughly
60°N latitude blow from the northeast; these are
called the polar northeasterlies, since we always
name wind based on the direction it is blowing from. The prevailing winds (near mean sea level)
from 90°S latitude (the south pole) to roughly 60°S latitude blow from the southeast; these are called the
polar southeasterlies, since we always name wind
based on the direction it is blowing from.
Notice that air sinks at roughly thirty degrees latitude in each
hemisphere, while air rises at roughly sixty degrees latitude in each
hemisphere. Sinking air is high-density,
high-pressure air, while rising air is low-density, low-pressure air. Therefore, we have high pressure at roughly
thirty degrees latitude in each hemisphere, and we have low pressure at roughly
sixty degrees latitude in each hemisphere.
The pressure gradient force pushes air from high pressure toward low
pressure. Hence, wind will blow from roughly
thirty degrees latitude to roughly sixty degrees latitude in each hemisphere,
where the air rises and is pushed back to thirty degrees latitude where it
sinks, completing the midlatitude circulation
cells. However, the Coriolis force
causes rightward deflections in the northern hemisphere and leftward
deflections in the southern hemisphere.
The net result of the pressure gradient force together with the Coriolis
force is that the prevailing winds (near mean sea level) from roughly 30°N latitude to roughly 60°N
latitude blow from the southwest; these are called the southwesterlies,
since we always name wind based on the direction it is blowing from. The prevailing winds (near mean sea level)
from roughly 30°S latitude to roughly 60°S latitude blow from the northwest; these are called the
northwesterlies, since we always name wind based on
the direction it is blowing from. To
summarize, there are three prevailing winds in each hemisphere, and there are
three circulation cells in each hemisphere.
In the northern hemisphere, the prevailing winds (near mean sea level)
are the northeast trade winds near the equator, the southwesterlies
at the midlatitudes, and the polar northeasterlies near the north pole. In the southern hemisphere, the prevailing
winds (near mean sea level) are the southeast trade winds near the equator, the
northwesterlies at the midlatitudes,
and the polar southeasterlies near the south pole. The
circulation cells are called the two Hadley cells (one in each hemisphere) near
equator, named for the British meteorologist George Hadley, the two Ferrel cells (one in each hemisphere) at the midlatitudes, named for the American meteorologist William Ferrel, and the two polar cells (one in each hemisphere)
near the poles. If the Earth rotated
faster, the Coriolis force would be stronger, thus causing more prevailing winds
and more circulation cells in each hemisphere.
If the Earth rotated slower, the Coriolis force would be weaker, thus
causing fewer prevailing winds and fewer circulation cells in each hemisphere. If the Earth stopped rotating, the Coriolis
force would vanish, and there would be only one prevailing wind and only one
circulation cell in each hemisphere. A
spectacular example of the effects of a strong Coriolis force is the planet
Jupiter, which rotates more than twice as fast as the Earth. In fact, Jupiter is the fastest rotating
planet in the Solar System. Therefore,
Jupiter has the strongest Coriolis force out of all the planets in the Solar
System. This very strong Coriolis force
has divided Jupiter’s atmosphere into many prevailing winds and many
circulation cells. We can actually see
these winds in photographs of Jupiter.
We can even see these winds if we look at Jupiter with our own eyes
through a sufficiently powerful telescope.
At the equator, there is
little to no wind, since the air is rising; this is called
the equatorial low, since low-pressure, low-density air rises. This rising air expands adiabatically, cools,
and becomes more humid. The dew point may be achieved, forming clouds and rain. Indeed, there is a perpetual band of clouds
at the equator, and the perpetual rain from these clouds causes tropical
rainforests at and near the equator, such as the Amazon rainforest in northern
South America, the Congo rainforest in central Africa, and the Indonesian
rainforests. Sailing ships that found
themselves at the equatorial low would become stuck, since there are no winds
to push ships. For this reason, the
equatorial low is also called the doldrums. Sailors would pray that their ship happens to
drift slightly to the north or slightly to the south to catch one of the trade
winds that would push them again. At
least the sailors could drink the perpetual rainwater while stuck at the
equatorial low (the doldrums). As we
discussed earlier in the course, the lack of wind at the equatorial low permits
the equatorial countercurrents to flow virtually unhindered against the
direction of other surface ocean currents near the equator. At roughly thirty degrees latitude in both
hemispheres, there is also little to no wind, but for the opposite reason. The air is sinking; these are
called the subtropical highs, since high-pressure, high-density air
sinks. This sinking air contracts
adiabatically, warms, and becomes less humid (more dry). Hence, we do not have clouds or rain. Indeed, there is a perpetual band of clear
skies free of clouds at roughly thirty degrees latitude in both
hemispheres. The
perpetual lack of rain causes hot deserts at and near roughly thirty degrees
latitude in both hemispheres, such as the Basin and Range in southwestern
United States and northwestern Mexico (including the Mojave Desert, the Sonoran
Desert, and the Chihuahuan Desert), the Sahara in
northern Africa, the Arabian Desert in the Arabian peninsula, the Gobi in China
and Mongolia, the Patagonian Desert in Argentina, the Kalahari in southern
Africa, and the Great Australian Desert in Australia (including the Great
Victoria Desert, the Great Sandy Desert, the Tanami Desert, the Simpson Desert,
and the Gibson Desert). Sailing
ships that found themselves at the subtropical high in either hemisphere would
become stuck, since there are no winds to push ships. There would also be no rain for the sailors
to drink. Therefore, not only would sailors
pray that their ship happens to drift slightly to the north or slightly to the
south to catch prevailing winds that would push them again, but the sailors
would also kill their horses to stretch out their limited supply of drinking
water. For this reason, the subtropical
highs are also called the horse latitudes. At roughly sixty degrees latitude in both
hemispheres, there is little to no wind, since the air is rising; these are called the subpolar lows, since low-pressure,
low-density air rises. This rising air
expands adiabatically, cools, and becomes more humid. The dew point may be
achieved, forming clouds and rain.
Indeed, there is a perpetual band of clouds at roughly sixty degrees
latitude in both hemispheres, and the perpetual rain from these clouds causes
boreal forests (cold forests or taigas) at and near roughly 60°N
latitude, including the Canadian boreal forests, the Scandinavian boreal
forests, and the Russian boreal forests.
Theoretically, there would be boreal forests (cold forests or taigas) at
and near roughly 60°S latitude if there were land at
these latitudes. At the poles, there is
also little to no wind since the air is sinking; these are called the polar
highs, since high-pressure, high-density air sinks. This sinking air contracts adiabatically,
warms, and becomes less humid (more dry).
Hence, we do not have clouds or rain.
Indeed, there is a perpetual area free of clouds at and near the poles
in both hemispheres. To summarize, we
have the equatorial low (the doldrums) at the equator, we have the subtropical
highs (the horse latitudes) at roughly thirty degrees latitude in both
hemispheres, we have the subpolar lows at roughly sixty degrees latitude in
both hemispheres, and we have the polar highs at ninety degrees latitude in
both hemispheres. At the lows, we have
rising air, causing humid climates from perpetual clouds and rain. At the highs, we have sinking air, causing
arid (dry) climates from the perpetual absence of clouds and rain. Actually, we expect arid climates at the
highs for an additional reason: any moisture that was in the air precipitated
out of the rising air at the equatorial low and at both subpolar lows before being pushed toward the highs where the air sinks. With moisture subtracted and in addition
warming temperatures from sinking air, we expect extremely arid (dry) climates
at both subtropical highs and at both polar highs.
An air mass is an enormous
mass of air that has roughly the same temperature and pressure throughout its
volume at a given elevation. We can
classify air masses based on their temperature.
An air mass that forms near the equator will be warm; these are called tropical air masses, which we label with the
uppercase (capital) letter T for tropical.
An air mass that forms near the poles will be cold; these are called polar air masses, which we label with the
uppercase (capital) letter P for polar.
We can also classify air masses based on their moisture. An air mass that forms over the ocean or any
body of water will be humid, since evaporating water will add moisture to the
air mass; these are called maritime air masses, which
we label with the lowercase letter m for maritime. An air mass that forms over a continent will
be dry, since there is little water on the continent to evaporate to add
moisture to the air mass; these are called continental
air masses, which we label with the lowercase letter c for continental. To summarize, there are four different types
of air masses. An air mass that forms
over a body of water near the equator will be humid and warm; these are called maritime tropical air masses, which we label with
the symbol mT.
An air mass that forms over a body of water near the poles will be humid
and cold; these are called maritime polar air masses,
which we label with the symbol mP. An air mass that forms over a continent near
the equator will be dry and warm; these are called
continental tropical air masses, which we label with the symbol cT. Finally, an air
mass that forms over a continent near the poles will be dry and cold; these are called continental polar air masses, which we label with
the symbol cP.
We must emphasize that once an air mass is born of a certain type, it
does not remain that type permanently.
In other words, the particular type of an air mass can change. For example, an air mass that forms near the
equator will be warm; it will be a tropical air mass. If this air mass happens to move toward one
of the poles, it may become colder and colder until we must reclassify it as a
polar air mass. The reverse can
occur. An air mass that forms near one
of the poles will be cold; it will be a polar air mass. If this air mass happens to move toward the
equator, it may become warmer and warmer until we must reclassify it as a
tropical air mass. As another example,
an air mass that forms over a continent will be dry; it will be a continental
air mass. If this air mass happens to
move over the ocean or any body of water, it may become more and more humid as
evaporating water adds more and more moisture to the air mass. Eventually, we must reclassify it as a
maritime air mass. The reverse can
occur. An air mass that forms over the
ocean or any body of water will be humid; it will be a maritime air mass. If this air mass happens to move over a
continent, it may lose more and more moisture through precipitation that will not be replenished, since there is little water on the
continent to evaporate. The air mass
becomes less and less humid until we must reclassify it as a continental air
mass. A spectacular example of the
changing of an air mass is lake-effect snow.
The five Great Lakes are between the United States and Canada, two
countries in the North American continent.
Cities on the windward side of the Great Lakes may experience very
little snow, since air masses that form over either Canada or the United States
would be continental air masses (dry air masses). However, a city on the leeward side (the
opposite side) of the Great Lakes may experience enormous amounts of snow. This is because a continental air mass that
moves over the Great Lakes will become more and more humid as water evaporates
from the Great Lakes. By the time the
air mass has crossed the Great Lakes, the air mass has become so humid that it
is now a maritime air mass. The humid
maritime air mass then precipitates snow onto these cities on the opposite side
of the Great Lakes from cities that experienced no snow from the same air mass
when it was formerly a dry continental air mass before crossing the Great
Lakes. As a result, two cities that are
not particularly distant from each other may nevertheless experience vastly
different amounts of precipitation, since these two cities are on two opposite
sides of a large body of water.
The Bjørgvin Theory of Meteorology
The fundamental
theory of meteorology was formulated by the Norwegian meteorological physicist Vilhelm Bjerknes and other
meteorologists in Bjørgvin, Norway. Consequently, we will refer to the
fundamental theory of meteorology as the Bjørgvin
Theory of Meteorology. According to the Bjørgvin Theory of Meteorology, the Earth’s troposphere
(the lowest layer of the atmosphere) is divided into
many pieces called air masses. These air
masses are pushed by the prevailing winds, and much
meteorological activity (commonly known as weather) occurs at the
boundary between two air masses, which is called a front. This Bjørgvin
Theory of Meteorology, the fundamental theory of meteorology, is remarkably
similar to the Theory of Plate Tectonics, the fundamental theory of
geology. As we discussed earlier in the
course, the Theory of Plate Tectonics states that the Earth’s lithosphere (the
uppermost layer of the geosphere) is divided into many
pieces called tectonic plates. These
tectonic plates are pushed by convection cells in the asthenosphere (underneath
the lithosphere), and much geological activity occurs at the boundary between
two tectonic plates. These two
fundamental theories have further similarities.
Just as there are different types of tectonic plate boundaries that
cause different types of geological activities as we discussed earlier in the
course, there are different types of fronts that cause different types of
meteorological activities (commonly known as weather). A cold air mass pushing on a warm air mass is called a cold front.
The symbol for a cold front on a weather map is triangles along the
front pointing in the direction in which the cold front is moving. A warm air mass pushing a cold air mass is called a warm front.
The symbol for a warm front on a weather map is semicircles along the
front again pointing in the direction in which the warm front is moving. Cold fronts move faster than warm fronts, as
we will discuss. Therefore, a
faster-moving cold front can catch up to and merge with a slower-moving warm
front. This is called
an occluded front. We will discuss the meaning
of the term occluded shortly. The symbol
for an occluded front on a weather map is both triangles and semicircles along
the front again pointing in the direction in which the occluded front is
moving. A front that does not move for
several days or perhaps even a couple of weeks is called
a stationary front. The symbol for a
stationary front on a weather map is both triangles and semicircles along the
front, but the triangles and the semicircles point in two opposite directions.
The meteorological term front
is borrowed from military terminology. Vilhelm Bjerknes and other meteorologists formulated the Bjørgvin Theory of Meteorology during and shortly after the
Great War (commonly known as the First World War or World War I) roughly one
hundred years ago. The Great War (the
First World War or World War I) was the most global and most horrific war in
human history up to that time, compelling many people throughout the world to often draw military analogies. A military front is the boundary between two
opposing armies. If one army advances
over (or pushes) the other army, the military front will move with the
advancing army. If two armies are equally matched, the military front will not move. This is called a
stationary military front, the textbook example being the western front of the
Great War (the First World War or World War I).
The western front remained stationary for most of the years of the Great
War since the combined British and French armies on the western side of the
western front equally matched the German army on the eastern side of the
western front. The western front did not
move until the United States joined the British and the French toward the end
of the Great War. The combined British,
French, and American armies now had sufficient momentum to advance upon the
German army, pushing the western front to the east. As Vilhelm Bjerknes and other meteorologists formulated the Bjørgvin Theory of Meteorology during and shortly after the
Great War, they imagined air masses pushing each other as if they were opposing
armies. It is for this reason that
meteorologists to the present day refer to the boundary between two air masses
as a front.
Since warm air rises and cold
air sinks, the actual front between two air masses is not a perfectly
vertical wall. The actual front
between the two air masses is an inclined wall, since the rising warm air will
be above the sinking cold air. In other
words, the sinking cold air will be below the rising warm air. Therefore, a cold front is inclined backwards
as the cold air mass pushes the warm air mass, while a warm front is inclined
forwards as the warm air mass pushes the cold air mass. Moreover, since cold air is more dense than warm air, the cold air mass can strongly
push the warm air mass, making the cold front more vertical than a warm
front. In other words, a warm front is more shallow than a cold front. Since warm fronts are more
shallow, it takes a longer duration of time for a warm front to move
over any particular location. Since cold
fronts are more vertical, it takes a shorter duration
of time for a cold front to move over any particular region. Along both cold fronts and warm fronts,
rising hot air will expand adiabatically and cool thus becoming more humid; the
dew point could be achieved, causing clouds and
possibly precipitation along the front.
Since a warm front is more shallow, all of the
precipitation will be spread over a larger area; consequently, the
precipitation along a warm front is often mild.
The usual weather associated with a warm front is gentle precipitation
over a long duration of time (often many hours) followed by warmer temperatures
as compared with the temperatures before the warm front arrived. Since a cold front is more
vertical, all of the precipitation will be concentrated over a smaller
area; consequently, the precipitation along a cold front is often severe. The usual weather associated with a cold
front is intense precipitation over a brief duration of time (often only a few
minutes), followed by colder temperatures as compared with the temperatures
before the cold front arrived.
As a concrete application of
the Bjørgvin Theory of Meteorology, consider weather
patterns in the contiguous United States, which is at the midlatitudes
of the northern hemisphere. The
prevailing winds of the midlatitudes of the northern
hemisphere are the southwesterlies. Therefore, weather patterns (both good
weather and bad weather) are pushed from the west
toward the east by these southwesterlies. This explains why weather patterns move
across the United States from the west toward the east; the weather patterns
are pushed by the southwesterlies. Philadelphia is west of New York City, and
Chicago is further west from Philadelphia.
A weather pattern in Chicago will move from Chicago toward Philadelphia,
and the weather pattern will continue to move from Philadelphia toward New York
City. Now consider a low
pressure system being pushed from west to east by these southwesterlies.
Winds will blow inward toward this moving low pressure
system. Winds from the south will carry
warmer air, since they are from more tropical latitudes. Winds from the north will carry cooler air,
since they are from more polar latitudes.
The Coriolis force deflects all of these winds to the right ultimately
circulating them counterclockwise around this moving low pressure system, since
the United States is in the northern hemisphere. Hence, the warmer winds from the south will be deflected to the east and will collide with cooler
air. Warm air pushing cold air is a warm
front; hence, there will be a warm front to the east of the moving low pressure system. Also, the cooler winds from the north will be deflected to
the west and will collide with warmer air.
Cool air pushing warm air is a cold front; hence, there will be a cold
front to the west of the moving low pressure
system. As this low
pressure system is pushed by the southwesterlies,
any given geographical region of the United States will first be attacked by
the warm front, often bringing many hours of gentle precipitation followed by
warmer temperatures. Then, the same geographical region will be attacked by the cold front, often
bringing only a few minutes of intense precipitation followed by colder
temperatures. Since cold fronts
move faster than warm fronts, the cold front to the west of the low pressure system may catch up to and merge with the warm
front to the east of the low pressure system, forming an occluded front. The cold air to the west of the former cold
front merges with the cold air to the east of the former warm front, thus
squeezing the warm air between them and wedging it upward, since warm air
rises. Hence, the formation of the
occluded front begins the dispersion of the entire low
pressure system. This is the
reason these fronts are called occluded; in colloquial
English, to occlude means to stop or to obstruct or to close.
As rising air and sinking air
rub against each other, electrons are transferred from
one thermal to another. This may create
an electric field between the clouds and the ground. Usually, air is a poor conductor of
electricity; air is usually an electrical insulator. However, all electrical insulators will conduct
electricity if subjected to electric fields of sufficiently
enormous strength. The threshold
electric field at which an electrical insulator becomes an electrical conductor
is called the dielectric breakdown of the
material. The dielectric breakdown of
air is roughly three million volts per meter.
If the electric field in air exceeds roughly three million volts per
meter, the air actually becomes an electric conductor. In other words, electrons can flow between
the clouds and the ground. This flow of
electrons is called lightning. There is an enormous quantity of energy associated
with lightning. Some of this energy is transferred to the air itself, causing a loud, explosive
sound called thunder. In brief,
lightning causes thunder. The light from
the lightning propagates at the speed of light, which is almost one million
times faster than the speed of sound. In
other words, sound propagates almost one million times slower than the speed of
light. The speed of light is so fast
that we never notice its propagation in our daily experiences; light seems to
propagate instantaneously fast. However,
sound propagates sufficiently slow that we notice its propagation in some of
our daily experiences. For example, some
of us notice while sitting near the outfield of a baseball stadium that there
is a delay between seeing and hearing a baseball bat crack a baseball. Some of us notice while sitting near the
infield of a baseball stadium that there is a delay between seeing and hearing
a baseball land in the baseball mitt of an outfielder. Some of us notice that there is a delay between
seeing and hearing a hockey stick strike a hockey puck. In all such examples, we see the event first,
then we hear the event second. Again,
light seems to propagate instantaneously fast, while sound propagates slow
enough that the sound arrives noticeably after the light. The speed of sound through air is roughly one
mile per five seconds. We may state this
speed as roughly five seconds per mile.
We can use this relatively slow propagation of sound to estimate how far
away a storm is occurring from our location.
We simply count the number of seconds starting from when we see
lightning until we hear thunder. For
every five seconds we count, the storm is roughly one mile distant. For example, if we see lightning and count
fifteen seconds until we hear thunder, the storm is roughly three miles
distant, since every five seconds we counted corresponds to roughly one mile of
distance. If we count many seconds after
seeing lightning but never hear thunder, this means that the storm is very far
away. Thunder always propagates outwards
in all directions, spreading its total energy thinner and thinner. By the time the thunder arrives at our
location, the sound energy was too diluted for our
ears to hear. At the opposite extreme,
suppose we see lightning and immediately thereafter we hear thunder; in other
words, suppose we did not have the opportunity to count to even one second
before hearing thunder. This means that
the storm is very close; we are probably right in the middle of the storm.
A tornado is a continental
storm with fast, circulating winds around an extremely low-pressure
thermal. Most tornadoes are a few dozen
meters across. A large tornado could be
a couple hundred meters across. Enormous
tornados that are one kilometer across are very rare. Most of the tornadoes in the world occur in
the midwestern United States. This is because cP
air masses (continental polar air masses) form over Canada, since Canada is in
the North American continent and is near the north pole,
while cT air masses (continental tropical air masses)
form over Mexico, since Mexico is also in the North American continent but near
the equator. Moreover, there are two
mountain ranges along both coasts of North America: the Rocky Mountains along
the Pacific coast (the west coast) and the Appalachian Mountains along the
Atlantic coast (the east coast). These
two mountain ranges tend to confine air masses between them. Hence, cP air
masses that form over Canada and cT air masses that
form over Mexico tend to collide over the country that is between Canada and
Mexico; that country is the United States.
For all these reasons, most of the tornadoes in the entire world occur
in the midwestern United States.
The Fujita scale (or F-scale)
is a tornado wind-speed scale, named for the Japanese-American meteorologist
Tetsuya Theodore Fujita who formulated this scale. The weakest tornadoes are designated F0. More powerful
than F0 would be called F1
followed by F2, F3, and F4. The most
powerful tornados are designated F5. Even an F0 tornado
is powerful enough to destroy entire towns; many people have
been killed by F0 tornados, the weakest scale
of tornado. We must always seek shelter
during a tornado warning, regardless of the Fujita-scale number of the tornado.
The largest storms in the
entire world form from low-pressure mT air masses
(maritime tropical air masses). These
storms are called hurricanes if they form in the
Atlantic Ocean, and they are called typhoons if they form in the Pacific
Ocean. Other than their oceanic
location, there is no difference between a hurricane and a typhoon. The development of a hurricane/typhoon is as
follows. A slightly low-pressure mT air mass is called a tropical
disturbance. If a tropical disturbance
happens to form at or near the equator, the Coriolis force will be too weak to
cause any circulation of winds, and the tropical disturbance will quietly
disperse. However, if a tropical
disturbance happens to form significantly north or south of the equator, the
Coriolis force may be strong enough to circulate the winds. When the winds are sufficiently strong, the
tropical disturbance becomes a tropical depression. On rare occasions, the winds are so strong
that the tropical depression may become a tropical storm. At this point, the tropical storm is given a human name, as we will discuss. On very rare occasions, the winds become so
extraordinarily strong that the tropical storm becomes a
hurricane/typhoon. In this case, the hurricane/typhoon
retains its tropical-storm human name, as we will discuss. To summarize, first we have a tropical
disturbance, then we have a tropical depression, then we have a tropical storm,
then we have a hurricane/typhoon.
Since a hurricane/typhoon is
a low-pressure system, the winds in a hurricane/typhoon circulate
counterclockwise in the northern hemisphere but circulate clockwise in the
southern hemisphere. The winds circulate
around the eye of the hurricane/typhoon, where ironically there is calm weather
and clear skies. When a northern-hemisphere
hurricane/typhoon attacks a continent, the winds to the right of the eye push
the ocean waters onto the continent.
This is called the storm surge. The winds to the left of the eye push the
ocean waters away from the continent; thus, there is no storm surge to the left
of the eye. Therefore, most of the
destruction to the left of the eye is from the winds themselves. To summarize, most of the devastation from a
northern-hemisphere hurricane/typhoon is from the storm surge to the right of
the eye, but most of the devastation from a northern-hemisphere
hurricane/typhoon is from the winds to the left of the eye. These directions are
reversed in the southern hemisphere, but note that hurricanes/typhoons
are rare in the southern hemisphere.
This is because winds would circulate clockwise around
hurricanes/typhoons in the southern hemisphere, causing the winds on the
southern edge of the storm to blow from the east. However, the prevailing winds at the midlatitudes of the southern hemisphere blow from the west,
thus weakening the storm winds and preventing tropical storms from
strengthening to hurricanes/typhoons. Of
course, we could present the same argument for the northern hemisphere. Winds circulate counterclockwise around
hurricanes/typhoons in the northern hemisphere, causing the winds on the
northern edge of the storm to again blow from the
east. Again, the prevailing winds at the
midlatitudes of the northern hemisphere blow from the
west, which should again weaken the storm winds and prevent tropical storms
from strengthening to hurricanes/typhoons.
Indeed, this is an important reason why the strengthening of tropical
storms to hurricanes/typhoons is rare even in the northern hemisphere. However, the Antarctic Circumpolar Current at
the midlatitudes of the southern hemisphere is the
strongest surface ocean current in the entire world, as we discussed earlier in
the course. Although the Antarctic
Circumpolar Current is caused by the prevailing winds at the midlatitudes of the southern hemisphere, this surface ocean
current is so strong that it actually pushes back on the atmosphere, making the
midlatitude prevailing winds in the southern
hemisphere stronger than the midlatitude prevailing
winds in the northern hemisphere. As a
result, the formation of hurricanes/typhoons in the southern hemisphere is much
less likely than the formation of hurricanes/typhoons in the northern
hemisphere. In either hemisphere, most
of the devastation inland from a hurricane/typhoon is from flooding from
rain. As we will discuss later in the
course, flooding is the most common and the most destructive of all natural
disasters.
The human name of a tropical
storm in the Atlantic Ocean is chosen from six lists
each containing twenty-one alphabetized human names. For example, the first tropical storm in the
Atlantic Ocean in the year 2017 was named tropical storm Arlene, the second was
named tropical storm Bret, the third was named
tropical storm Cindy, and so on and so forth.
Notice that the names are in alphabetical order. Although the English alphabet has twenty-six
letters, these six lists each have only twenty-one names because names
beginning with the five letters Q, U, X, Y, and Z are not
used, since human names beginning with any of these five letters are rare. If a tropical storm is
promoted to a hurricane, then it retains its human name. For example, tropical storm Franklin was promoted to hurricane Franklin in the year 2017. These six lists of human names are recycled
every six years. However, if a hurricane
is particularly destructive, then its name is permanently retired and is
forever associated with the hurricane for that particular year. A new human name beginning with the same
letter of the English alphabet must then replace that name for future years. For example, the fourth tropical storm in the
year 2013 should have been named Dean, but hurricane
Dean was so destructive in the year 2007 that the name Dean was permanently
retired and replaced with the name Dorian.
If there happens to be more than twenty-one tropical storms in the
Atlantic Ocean in any given year, then the letters of the Greek alphabet are used after reaching the end of the list of twenty-one
names. For example,
the twenty-second tropical storm in the Atlantic Ocean in the year 2005 was
named tropical storm Alpha, the twenty-third was named tropical storm Beta
(later promoted to hurricane Beta), the twenty-fourth was named tropical storm
Gamma, the twenty-fifth was named tropical storm Delta, the twenty-sixth was
named tropical storm Epsilon (later promoted to hurricane Epsilon), and so on
and so forth. There are other
lists of names for tropical storms in the Pacific Ocean. Again, there are twenty-one human names in
each list of names for the Atlantic Ocean, and there are twenty-four letters in
the Greek alphabet. Twenty-one plus
twenty-four equals forty-five. What do
we do if there are more than forty-five tropical storms in the Atlantic Ocean
in a single year? In this case, we run
to the nearest church, since it is probably the end of the world!
The Saffir-Simpson
scale is a hurricane/typhoon scale, named for American engineer Herbert Saffir and American meteorologist Robert Simpson who
together formulated this scale. The
weakest hurricane/typhoon is called Category 1, stronger
is called Category 2, even stronger is called Category 3, stronger is called
Category 4, and the strongest hurricane/typhoon is called Category 5. We must keep in mind that even a Category 1
hurricane/typhoon is stronger and more destructive than a tropical storm. For example, hurricane Sandy was Category 1
when it attacked and devastated New Jersey in the year 2012. Even tropical storms, which are themselves
weaker than Category 1 hurricanes/typhoons, can destroy entire towns; many
people have been killed by tropical storms, themselves
weaker than the weakest hurricanes/typhoons.
We must always seek shelter during a tropical storm warning, and we must
certainly always seek shelter during a hurricane/typhoon warning, regardless of
its Saffir-Simpson category number.
Climatology
The study of short-term
trends and variations in the atmosphere is called
meteorology, and someone who studies short-term trends and variations in the
atmosphere is called a meteorologist. By
short-term, we may mean a few minutes, a few hours, a few days, or a few
weeks. The study of long-term trends and
variations in the atmosphere is called climatology,
and someone who studies long-term trends and variations in the atmosphere is
called a climatologist. By long-term, we
may mean months, years, decades, centuries (hundreds of years), millennia
(thousands of years), or even millions of years. The best term for the study of the atmosphere
(short-term and/or long-term) is called atmospheric
sciences, and someone who studies the atmosphere (short-term and/or long-term)
should be called an atmospheric scientist.
Statistics is
used to study trends and variations of any kind. Any collection of numbers is
called data, and the purpose of statistics is to calculate two
quantities about data: the central tendency of the data and the dispersion of
the data. The central tendency of the
data is a number that is a typical representative of most of the data. The most common way of measuring central
tendency is the average, which we will call the mean. The mean of the data is the sum of the
numbers divided by the number of numbers.
The most common way of measuring dispersion is the standard deviation,
but in this course we will measure dispersion with the
range. The range of the data is the
difference between the largest number and the smallest number. In atmospheric sciences, the daily
temperature mean is the average of the hottest temperature and the coldest
temperature in any given day. For
example, if the hottest temperature today is eighty degrees fahrenheit
and if the coldest temperature today is seventy degrees fahrenheit,
then the daily temperature mean for today is seventy-five degrees fahrenheit, since eighty plus seventy is one hundred and
fifty, and dividing this by two yields seventy-five. The daily temperature range is the difference
between the hottest temperature and the coldest temperature in any given
day. In the previous example, the daily
temperature range for today would be ten fahrenheit
degrees, since eighty minus seventy equals ten.
The monthly temperature mean is the average of all the daily means for
that month. For example, if a particular
month happens to have thirty days, the monthly temperature mean for that month
would be the sum of all the daily means for that month divided by thirty. The monthly temperature range is the
difference between the hottest daily mean and the coldest daily mean during
that month. The annual temperature mean
is the average of all the monthly means for that year. In other words, the annual temperature mean
is the sum of all the monthly means for that year divided by twelve, since
there are twelve months in one year. The
annual temperature range is the difference between the hottest monthly mean (almost always July or August in the northern hemisphere) and
the coldest monthly mean (almost always January or February in the northern
hemisphere) of that year.
Generally, temperature means
are hotter at and near the equator, while temperature means are colder at and
near the poles. At the midlatitudes, temperature means are hotter during summers
and colder during winters. However, we
must not only specify trends and variations in the temperature, but trends and
variations in the precipitation must also be specified
in any climatological analysis.
Generally, precipitation means are high at and near the equator due to
the equatorial low (the doldrums).
Precipitation means are low at and near roughly thirty degrees latitude
in both hemispheres due to the subtropical highs (the horse latitudes). Precipitation means are high at and near
roughly sixty degrees latitude in both hemispheres due to the subpolar
lows. Finally, precipitation means are
low at and near the poles due to the polar highs.
Temperature ranges are
smaller at coasts and shores due to the marine effect: the oceans stabilize
temperatures due to the relatively large heat capacity of water. Conversely, temperature ranges are larger
inland due to the continental effect: continents do not stabilize temperatures
due to the relatively small heat capacity of land. In other words, inland winters tend to be
colder and inland summers tend to be hotter as compared with coasts and shores
where both winters and summers tend to be relatively mild. Extending this logic across the entire
planet, temperature ranges are generally smaller in the water hemisphere (the
southern hemisphere), since the abundance of southern-hemisphere
oceans stabilize temperatures in that hemisphere. Conversely, temperature ranges are generally
larger in the land hemisphere (the northern hemisphere), since the abundance of
northern-hemisphere continents do not stabilize
temperatures in that hemisphere. In
other words, winters in the northern hemisphere tend to be colder and summers
in the northern hemisphere tend to be hotter as compared with the southern
hemisphere, where both winters and summers tend to be relatively mild.
For most of the history of
planet Earth, the hot temperatures at the Equator and the cold temperatures at
the poles have been moderated by the oceans due to the
relatively large heat capacity of water.
However, the moving tectonic plates of the lithosphere slowly change the
configuration of the continents and the oceans over enormous timescales (millions
of years). If there happens to be
relatively isolated continents and/or microcontinents at the poles, the
relatively small heat capacity of these landmasses will permit the temperatures
at the poles to become extremely cold.
The result is an ice age, an extremely long period of time (millions of
years) when the temperature at the poles is so cold that enormous icecaps cover
these landmasses. To our knowledge,
there have been only five ice ages throughout the entire history of the Earth,
each lasting many millions of years. As
we discussed earlier in the course, the Current Ice Age began roughly thirty
million years ago when South America ripped off of
Antarctica, completely isolating Antarctica at the South Pole and establishing
the Antarctic Circumpolar Current surrounding Antarctica. Hence, Antarctica became extremely cold, and
our entire planet Earth plunged into the Current Ice Age. The Current Ice Age began roughly thirty
million years ago and continues to the present day. The Current Ice Age will last many more
millions of years as long as Antarctica remains isolated at the South Pole
surrounded by the Antarctic Circumpolar Current that further isolates
Antarctica. There are
only two scenarios that can end the Current Ice Age. In one scenario, Antarctica may move off of the South Pole, which would make it less cold. This would also interrupt the Antarctic
Circumpolar Current, which would contribute to warming temperatures. In the other scenario, another continent may
move to the South Pole and collide with Antarctica. This would end the isolation of Antarctica,
making it less cold. This would also
interrupt the Antarctic Circumpolar Current, again contributing to warmer
temperatures. Whether Antarctica moves
away from the South Pole or another continent moves toward the South Pole, it
takes millions of years for tectonic plates to move significantly, as we
discussed earlier in the course.
Therefore, the Current Ice Age will continue for millions of more years
to come. During the Current Ice Age, the
southern icecap covers the continent Antarctica, and the northern icecap covers
the microcontinent Greenland.
Within the
Current Ice Age, there have been many periods of time when the Earth has become
even colder. These are glacial periods of the Current Ice
Age. Between two glacial periods is an
interglacial period when the Earth is not as cold. The Earth becomes so cold during a glacial
period that the icecaps expand beyond the poles and advance onto other
continents. A major glacial period lasts
roughly one hundred thousand years, while a minor glacial period lasts between
roughly twenty-five thousand years and roughly fifty thousand years. We are currently within an interglacial
period of the Current Ice Age. This
interglacial period began roughly twelve thousand years ago at the end of a
major glacial period that lasted roughly one hundred thousand years. Plate tectonics cannot be responsible for the
alternation between glacial periods and interglacial periods within the Current
Ice Age, since tectonic plates do not move appreciably over timescales of
thousands of years. The alternation
between glacial periods and interglacial periods within the Current Ice Age is caused by the Milanković
cycles, named for the Serbian climatologist and astronomer Mulutin
Milanković who formulated this theory. The Earth’s orbit around the Sun is presently
almost a perfect circle. In other words,
the eccentricity of the Earth’s orbit around the Sun is close to zero, but this
has not always been the case nor will it always be the case. Gravitational perturbations (tugs) from the
other planets, primarily Jupiter, change the eccentricity of the Earth’s
orbit. When the Earth’s orbit is
perturbed to become more elliptical, the Earth will be
significantly further from the Sun, causing the Earth to become
significantly colder thus causing a major glacial period of the Current Ice
Age. Calculations show that the
eccentricity of the Earth’s orbit changes once every one hundred thousand years
(roughly), which is roughly the duration of time of a major glacial period
within the Current Ice Age. Minor glacial periods are caused by the precession and the nutation
of the Earth’s rotational axis.
Precession is the turning of an axis around another axis. Nutation is the nodding of an axis resulting
in a change in obliquity. The Earth’s
rotational axis precesses and nutates, and this changes
the amount of sunlight the Earth receives from the Sun, which in turn causes
minor glacial periods within the Current Ice Age. Calculations show that the Earth’s rotational
axis precesses once every twenty-six thousand years
(roughly), and the Earth’s rotational axis nutates
once every forty-one thousand years (roughly).
These are roughly equal to the duration of time of minor glacial periods
within the Current Ice Age.
Long-term variations in
global temperature (over millions of years) are caused by slowly moving
tectonic plates, resulting in ice ages when continents and/or microcontinents
happen to be relatively isolated at or near the poles, as with the Current Ice
Age. Intermediate-term variations in
global temperature (over thousands of years) cause glacial periods and
interglacial periods within the Current Ice Age. These intermediate-term variations in global
temperature (over thousands of years) are caused by
the Milanković cycles (the variations of the
eccentricity of the Earth’s orbit around the Sun, the precession of the Earth’s
rotational axis, and the nutation of the Earth’s rotational axis). However, there are also short-term variations
in global temperature, from centuries to decades, and even as short as a few
years. Extremely short-term variations
in global temperature (over a few years) are caused by
violent igneous eruptions. As we
discussed earlier in the course, a single igneous eruption can cause global
cooling for a few years, since igneous eruptions eject ash into the atmosphere,
reducing the amount of incoming sunlight to the Earth. Short-term variations in global temperature
(from decades to centuries) are caused by a
combination of the Pacific Decadal Oscillation (PDO),
the Atlantic Multidecadal Oscillation (AMO), and variations in solar activity. As we discussed earlier in the course, the
Pacific Decadal Oscillation (PDO) is the alternation
between decades of El Niño domination in the Pacific Ocean and decades of La
Niña domination in the Pacific Ocean.
The Atlantic Multidecadal Oscillation (AMO) is a similar oscillation in the Atlantic Ocean, as we
also discussed earlier in the course.
Although direct observations of the PDO and
the AMO only stretch back several decades,
measurements of the radioactive isotope carbon-fourteen 
within trees have revealed that the PDO and
the AMO each undergo roughly sixty-year cycles, where one complete PDO for
example consists of roughly three decades of El Niño domination followed by
roughly three decades of La Niña domination.
Quantitative observations of solar activity stretch back a few
centuries, giving us a stronger understanding of how variations in solar
activity affect global temperatures. At
times, the Sun is more active, radiating more energy that warms planet Earth;
at other times, the Sun is more quiet, radiating less
energy that cools planet Earth. These
variations in solar activity manifest themselves through sunspots, regions on
the surface of the Sun where magnetic field strengths are particularly
strong. These sunspots have been directly observed for roughly four hundred years,
since the invention of the telescope.
Astronomers have observed that the number of sunspots goes through a
roughly eleven-year cycle. In one
complete cycle, the number of sunspots increases then decreases over a time period of roughly eleven years. For example, the Sun experienced a period of
increasing activity during the last few years of the twentieth century and the
first few years of the twenty-first century (the current century). This period of increasing solar activity was followed by a period of decreasing solar activity. During that particular period of decreasing
solar activity, some of the coldest monthly temperature means over the last one
hundred years occurred. Furthermore,
measurements of the radioactive isotope carbon-fourteen within trees have revealed that this roughly
eleven-year solar cycle itself goes through a roughly two-hundred-year
cycle. This is the de Vries cycle, named for the Dutch physicist Hessel de Vries, one of the pioneers of radiocarbon dating. According to the de Vries
cycle, the Sun gradually increases in activity to what is
called a solar maximum then gradually decreases in activity to what is
called a solar minimum. Caution: the
eleven-year solar cycles continue to occur throughout each two-century de Vries cycle. Since
one complete de Vries cycle lasts for roughly two
centuries, each solar maximum and each solar minimum lasts for roughly one
hundred years. Over the past twelve
thousand years (since the beginning of the current interglacial period of the
Current Ice Age), there have been roughly sixty complete de Vries
cycles, with each de Vries cycle having one solar
maximum and one solar minimum. The
Modern Maximum occurred throughout most of the twentieth century, and the
Modern Minimum began toward the beginning of the twenty-first century (the
current century). These de Vries cycles have caused variations in global temperatures
over the past several thousand years.
For example, a solar maximum contributed to the Roman Warm Period,
lasting from the ancient Late Roman Republic to the ancient Early Roman Empire. As another example, another solar maximum
contributed to the Medieval Warm Period during the High Middle Ages. As yet another
example, a solar minimum contributed to the Little Ice Age, lasting from the
Late Middle Ages to the Early Modern Ages.
Most recently, the Modern Maximum that occurred throughout most of the
twentieth century contributed to the warming temperatures of that century, and
the Modern Minimum that began toward the beginning of the twenty-first century
(the current century) has already caused cooling temperatures that will
continue for the rest of the current century.
By combining the roughly sixty-year PDO, the
roughly sixty-year AMO, the roughly eleven-year solar
cycle, and the roughly two-century de Vries cycle, we
obtain a nearly perfect model of global temperature variations over the past
couple of thousand years. As we just
discussed, this model predicts that there will be gradual global cooling during
the current century. Caution: an unexpected
violent igneous eruption may cause additional global cooling in addition to
these cyclic variations.
To summarize the climate of
planet Earth, the average temperature of the atmosphere is determined primarily
by the Earth’s distance from the Sun together with the concentration of
greenhouse gases in the atmosphere, primarily water vapor, which warms the
Earth sufficiently so that it is habitable for life. Temperature means are hot near the equator,
temperature means are cold near the poles, and temperature means vary at the midlatitudes based on the seasons (warmer in the summertime
and cooler in the wintertime). The
abundance of liquid water that covers the Earth (the oceans) stabilizes global
temperatures due to the large heat capacity of water. Temperature ranges are smaller in the
southern hemisphere (the water hemisphere), while temperature ranges are larger
in the northern hemisphere (the land hemisphere). Temperature ranges are smaller at coasts and
shores due to the marine effect, while temperature ranges are larger inland due
to the continental effect. Precipitation means are high at and near the equator due to the
equatorial low (the doldrums), precipitation means are low at and near roughly
thirty degrees latitude in both hemispheres due to the subtropical highs (the
horse latitudes), precipitation means are high at and near roughly sixty
degrees latitude in both hemispheres due to the subpolar lows, and
precipitation means are low at and near the poles due to the polar highs. Long-term variations in global temperature
(over millions of years) are caused by slowly moving tectonic plates, resulting
in ice ages when continents and/or microcontinents happen to be relatively
isolated at or near the poles, as with the Current Ice Age that began roughly
thirty million years ago and continues to the present day. Intermediate-term variations in global
temperature (over thousands of years) cause glacial periods and interglacial
periods within the Current Ice Age.
These intermediate-term variations in global temperature (over thousands
of years) are caused by the Milanković
cycles (the variations of the eccentricity of the Earth’s orbit around the Sun,
the precession of the Earth’s rotational axis, and the nutation of the Earth’s
rotational axis). We are currently in an
interglacial period of the Current Ice Age, and this interglacial period began
roughly twelve thousand years ago.
Short-term variations in global temperature (over a few decades or a few
centuries) are caused by the roughly eleven-year solar
cycle, the roughly two-century de Vries cycle, the
roughly sixty-year PDO, and the roughly sixty-year AMO. We just began a
century of gradual global cooling resulting from these short-term solar cycles
and oceanic cycles. Finally, extremely
short-term variations in global temperature (over a few years) may result from
powerful igneous eruptions.
Links
Libarid A. Maljian homepage at the Department of Physics at CSLA at NJIT
Libarid A. Maljian profile at the Department of Physics at CSLA at NJIT
Department of Physics at CSLA at NJIT
College of Science and Liberal Arts at NJIT
New Jersey Institute of Technology
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