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
Summer 2020
Fourth Examination lecture notes
Introduction to the Atmosphere
An atmosphere is a thin layer
of gas held to a planet by its gravity.
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, and we will discuss these trace gases 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 lightest atom in the entire universe, and helium is the
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 pull.
Thus, the Earth lost its primary atmosphere because its own gravity was
too weak to hold onto hydrogen and helium.
This also happened to 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 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; 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 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 gravity. 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; this is where the oceans came from. 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 happened on this planet that to our
knowledge did not happen 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 extracted carbon dioxide gas from the atmosphere and replaced 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 ago, planet
Earth attained the atmosphere we enjoy to this 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 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. The average air
pressure at sea level is called one atmosphere of pressure, and it is equal to
101325 pascals of pressure. One pascal of pressure
is one newton of force per square meter of area. The 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, average air pressure
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 invert a
long, narrow container, and insert it inverted into any liquid. The air pressure will push down on the liquid
and up the long, narrow column. If the
air pressure is greater than average, it will push down more strongly on the
liquid, thus pushing it further up the column, making the column of liquid
taller. If the air pressure is less than
average, it will push down more weakly on the liquid, thus pushing it not as
far up the 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 down on the liquid and up the long, narrow column. Most barometers use mercury as the liquid; at
normal air pressure, mercury will be pushed 760 millimeters (or 29.9 inches) up
the narrow column. Thus, average air
pressure 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 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. The temperature of the
Earth’s atmosphere is typically a maximum at sea level and becomes cooler with
increasing elevation within the troposphere.
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 think of the
tropopause as the boundary between the troposphere and the stratosphere, but
the tropopause is more correctly defined as the end of the troposphere. Now the temperature 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 warming 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 think of the stratopause
as the boundary between the stratosphere and the mesosphere, but the stratopause is more correctly defined as the end of the
stratosphere. Now the temperature
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 think of the mesopause as the boundary
between the mesosphere and the thermosphere, but the mesopause
is more correctly defined as the end of the mesosphere. Now the temperature 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. To summarize, the temperature typically
becomes cooler with increasing elevation within the troposphere and the
mesosphere, while the temperature typically becomes warmer with increasing
elevation within the stratosphere and the thermosphere.
It may seems reasonable to
ask for the precise elevation at which the Earth’s atmosphere ends and outer
space begins. However, this is in fact
an ill-defined question. The concentration
of gases becomes thinner and thinner with increasing elevation until we reach a
certain elevation at which the concentration of gases of the Earth’s atmosphere
matches the concentration of gases of the surrounding outer space. It is a common misconception that outer space
is a perfect vacuum; this is false.
There is in fact no such thing as a perfect vacuum. 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 thermopause. The tropopause is the end of the troposphere,
the stratopause is the end of the stratosphere, and
the mesopause is the end of the mesosphere. If there were an end of the thermosphere
(which would also be the end of the entire atmosphere), that would be called
the thermopause, but there is no well-defined thermopause.
Nevertheless, if we insist upon a boundary between the Earth’s
atmosphere and outer space, we may arbitrarily quote the elevation of the
tropopause, since the Earth’s gravity pulls roughly ninety percent of all the
air in the atmosphere down to 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 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 sea level. This is extremely thin 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 as a rough estimate for 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 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 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 produce 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; cellular respiration also explains why
humans and animals must exhale carbon dioxide.
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 trap 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
makes the temperature of planet Earth significantly warmer than it would have
been otherwise. 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 warms 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 vacuum of outer space for example.
We now realize that whenever we use the word light in everyday English,
we probably mean to use the term visible light, since this is the type of light
that our eyes can actually see. The
visible part of the Electromagnetic Spectrum is actually quite narrow. Nevertheless, the visible part of the
Electromagnetic Spectrum can be subdivided.
In order, the subcategories of the visible part 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 just
before 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
just after 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, and the X-rays from the Sun should kill us in fairly short order. Something must be shielding us from the Sun’s
far ultraviolet and from the Sun’s X-rays.
Our atmosphere provides these shields.
Oxygen is an atom, and 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 normal oxygen for example, and this is the form of
oxygen humans and all animals must inhale.
Notice this is the form of oxygen appearing in the cellular respiration
reaction written above. Whenever anyone
uses the word oxygen, they are not being clear.
Do they mean atomic oxygen O or do they mean molecular oxygen O2? They
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, 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.
To summarize all the ways the
Earth’s atmosphere keeps us alive, humans and animals would not be able to
inhale 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, making 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. Earlier in the course, we discussed that the
entire atmosphere would be ionized by the Sun’s solar wind without the Earth’s
magnetic field deflecting most of these protons and electrons 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.
We all 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 happen because of the distance planet Earth is 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 sometimes 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 measures how elongated it is.
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 only about
2.5 million kilometers (1.5 million miles) closer to the Sun than average. When the Earth is at aphelion (furthest from
the Sun), it is only about 2.5 million kilometers (1.5 million miles) further
from the Sun than average. These closer
or further distances may seem large, but the Earth is on average 150 million
kilometers (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 distance from the Sun causes the
seasons: the Earth is closest to the Sun in wintertime and furthest from the
Sun in summertime! The Earth’s
perihelion is roughly January 03rd every year, but early January is in
wintertime! The Earth’s aphelion is
roughly July 03rd every year, but early July is in summertime! We are not saying that the 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 we would all 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 we would all 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 statement
is that this happens already; every year as the Earth orbits the Sun on its
elliptical orbit, it moves roughly 2.5 million kilometers (1.5 million miles)
closer to the Sun at perihelion and roughly 2.5 million kilometers (1.5 million
miles) further from the Sun at aphelion, and these variation 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 explain 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 its 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. 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.
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 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
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. 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. As we
discussed in an earlier lecture, this is because the southern hemisphere is
mostly covered with water. Since water
has a large heat capacity, 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.
The moment when the Earth’s
northern hemisphere is titled 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.
The equinoxes are moments 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, commonly known as 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. 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 a 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.
As we discussed toward the
beginning of 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 tilt of
its rotational axis. 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 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
mentioned toward the beginning of 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 mentioned toward the beginning of
the course. The words Cancer and
Capricorn refer to astronomical constellations of the zodiac; the reason these
lines of latitude are named after 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. Primitive 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 to ourselves
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
primitive humans thousands of years ago.
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; the truth is that 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 happens 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 the vernal equinox (spring equinox) all the way to the autumn
equinox. The reverse happens 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
watched the Sun appear lower and lower in the sky after the summer solstice;
perhaps many primitive humans were scared that the Sun would continue to move
downward until it disappeared below the horizon. However, by simply paying attention every
year, we notice 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 the birth of Jesus Christ was seen by early Christians as
bringing more and more light into a spiritually dark world. As another example of a religious holiday,
Easter is tied to the vernal equinox (spring equinox). Easter is always the first Sunday after the
first Full Moon after the vernal equinox (spring equinox) every year.
A thermometer is a device to
measure temperature. A thermometer is
based on the principle of thermal expansion.
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 longer length at a hotter temperature. We draw marks at each of these lengths, and
we draw other marks between these two marks.
To determine the air temperature, we simply read off whichever mark the
end of object meets based on its length at that temperature. Unfortunately, most substances expand and
contract by very tiny amounts when their temperature changes. Hence, the marks are close together, making
differences in length difficult to measure.
However, mercury expands by quite a noticeable amount as it becomes warmer,
and mercury contracts by quite a noticeable amount as it becomes colder. Therefore, most thermometers use mercury,
since the marks are 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 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
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; the hottest air temperature occurs roughly a month
later in late July. Similarly, the
winter solstice is almost never the coldest day of the year; the coldest air
temperature occurs roughly a month later in late January. 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 may receive the most direct sunrays on the summer solstice around
June 21st, but it still takes another month for the air temperature 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 sometime in late August.
The northern hemisphere may receive 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 temperature
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 sometime in late February.
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 all of 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 a
measure of the frequency with which 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. Now imagine 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 the air on one side of the room
is at a higher air pressure for whatever reason, and 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 all the pressure in 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 curve connecting places 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 be purely perpendicular to
isobars. If the pressure gradient force
were not purely perpendicular to isobars, then we would be able to break the
force into two components: one component purely perpendicular to the isobars and
one component along the isobars.
However, the component along the isobars must be zero, as we just
argued. Therefore, the pressure gradient
force must be purely perpendicular to the isobars. The pressure gradient force does not have two
components; it only has one component that is purely perpendicular to the
isobars. Every point on an isobar is at
the same air pressure, but two different isobars are at two different
pressures. Suppose as we move from one
isobar to the next isobar, the pressure always drops by a definite amount such
as 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, and we will have 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, and we will have light winds. This is remarkable, since the pressure drop
from one isobar to the next 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 in one case, while the same ten millibar pressure drop is shallow causing light winds in
another case.
An anemometer is a device
that measures the velocity of wind (both the speed and the direction of
wind). A primitive anemometer is
essentially a wind vane together with a flag.
As the wind blows, it turns the wind vane 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 determines 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 that 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
be called a wind. A high number on the
Beaufort scale would be a strong wind, which is called a gale.
We have already learned
enough basic meteorology to understand some basic weather patterns. Imagine 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 is, 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; this is why it is called the sea breeze. We often feel this sea breeze while at the
beach in the daytime. It is a steady
wind blowing from the ocean toward the continent during the daytime. In the nighttime, the opposite occurs. Imagine 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 is, 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; this is why it is called the land breeze. Perhaps some of us have felt this land breeze
while at the beach in the nighttime. It
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. Imagine a mountain
and a valley. In the daytime, the Sun
warms the Earth. Hot air is less dense,
and so hot air will be buoyed upward by the surrounding air. This is why hot air rises. Therefore, wind will blow from the valley up
toward the 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;
this is why it is called the valley breeze.
Perhaps some of us have felt this valley breeze while on a mountain in
the daytime. It is a steady wind blowing
from the valley up toward the mountain during the daytime. In the nighttime, the opposite occurs. The Earth cools at night. Cold air is more dense, and so cold air will
sink into the surrounding air. This is
why cold air sinks. Therefore, wind will
blow from the mountain down into the 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; this is why it is called the mountain
breeze. Perhaps some of us have felt
this mountain breeze while in a valley in the nighttime. It 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.
If the Earth were not
rotating, the study of the Earth’s atmosphere would be much simpler than it
actually is. The pressure gradient force
would simply push wind from high pressure toward low pressure perpendicular to
isobars. However, the Earth is rotating,
and the rotation of the Earth causes gross complications in the Earth’s
atmosphere. The rotation of the Earth
causes a Coriolis force, which is an example of a fictitious force or a pseudoforce. A
fictitious force or a pseudoforce is a force that
does not actually exist; it only seems to exist in certain frames of
reference. If a frame of reference is
rotating, projectiles will appear to suffer from deflections. The projectiles are not really deflecting;
they actually continue moving in straight lines. The frame of reference is rotating, which seems
to cause projectiles to deviate from straight lines; in actuality, the trajectories
remain straight. This apparent
deflection in rotating frames of reference is called the Coriolis force. The Coriolis force appears to cause rightward
deflections in frames of reference rotating counterclockwise and 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. The Earth is
rotating from west to east. When viewed
from above the north pole, the northern hemisphere appears to be rotating
counterclockwise. When viewed from above
the south pole, the southern hemisphere appears to be rotating clockwise. Therefore, there is a Coriolis force on
planet Earth that appears to cause rightward deflections in the northern
hemisphere and 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. 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 from high pressure toward low pressure perpendicular to the isobars, 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 causes these
apparent deflections, wind will not blow directly from high pressure toward low
pressure; wind will not blow perfectly perpendicular to isobars.
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. Now suppose we 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 shortly. 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 shortly.
To summarize, 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 up by the surrounding air.
As an alternative argument, the low pressure is caused by hot
temperatures, and hot air must rise.
Regardless, 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 into the surrounding air.
As an alternative argument, the high pressure is caused by cold
temperatures, and cold air must sink.
Regardless, 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 decreases in accord
with the law of atmospheres. Therefore,
the low-pressure thermal expands as its own pressure pushes the surrounding lower-pressure
air out of the way. As the high-pressure
thermal at the center of an anticyclone sinks, the surrounding air pressure
increases in accord with the law of atmospheres. Therefore, the high-pressure thermal
contracts as the surrounding high-pressure air compresses it. To summarize, 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 and expands. Conversely, an anticyclone is the weather
pattern around a high-pressure thermal that sinks and contracts; upon reaching
sea level, 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 and 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 must push the surrounding gas out of the way. This requires work, and work is a form of
energy. The gas extracts this energy
from its own internal energy content, and so it must become 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
it. This work is added to the internal
energy content of the gas, and so it must become warmer. This is remarkable; we can change the
temperature of a gas without adding or extracting heat. We can prove this by performing these
experiments while the gas is wrapped in a thermal insulator, which would 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 something it
often becomes warmer, and when we extract heat from something it often becomes
cooler. Nevertheless, we have also
revealed circumstances where we can change the temperature of a gas without the
addition or extraction of heat. In fact,
it is possible for a gas to become warmer under certain circumstances when we
have extracted heat! It is also possible
for a gas to become cooler under certain circumstances when we have added
heat! Such examples truly persuade us
that heat and temperature are two completely different physical concepts. Therefore, we must use two completely
different words 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 must become
cooler; since the temperature is cooling, this is not an isothermal
process. A gas that contracts
adiabatically must become warmer; since the temperature is warming, this is not
an isothermal process either. Also note
that a process where there is no pressure change is called an isobaric process.
Air is a poor conductor of
heat. A beautiful illustration of this
fact is 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 its intense heat, our hands are not in any danger. 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 are forced to conclude
that the air between our hands and the hot charcoal is a poor thermal
conductor. Since air is such a poor
conductor of heat, we always assume thermals in the Earth’s atmosphere do not
gain or lose heat from their surroundings.
This is called the adiabatic approximation, and it is an excellent
approximation for most meteorological processes. We have already concluded that the air at the
center of a cyclone rises and expands.
According to the adiabatic approximation, we conclude that the
low-pressure thermal expands adiabatically.
If it expands adiabatically, then it must cool. We have already concluded that the air at the
center of an anticyclone sinks and contracts.
According to the adiabatic approximation, we conclude that the
high-pressure thermal contracts adiabatically.
If it contracts adiabatically, then it must warm. To summarize, 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; upon reaching sea level, the winds blow outward while
circulating clockwise in the northern hemisphere and counterclockwise in the
southern hemisphere.
Relative humidity is a concept
that everyone believes that they understand; in actually, almost no one
correctly understands this concept of relative humidity. Most people believe that the relative
humidity of the air is the amount of a moisture in the air. This is such a gross simplification of the
true 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. Place a lid upon a cup of water; a piece of
paper will serve as a satisfactory lid.
Liquid water will continuously evaporate into water vapor, but the lid
will confine the water vapor into a small space. When the confined air holds the maximum
amount of water vapor it is able to hold, some of the water vapor must condense
back into liquid water to provide room for additional liquid water to evaporate
into water vapor. Some of this water
will condense back into the rest of the liquid in the cup, but some of it will
condense as drops of water on the sides of the cup and even underneath the
lid. When the air holds the maximum
amount of moisture 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 hold the maximum moisture 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 the air is the amount of moisture in the
air as a fraction of the saturation amount at the given temperature. Let us spend some time to fully understand
this definition. Firstly, the relative
humidity of the air is directly related to the amount of moisture in the
air. Adding moisture to the air
increases the relative humidity, while subtracting moisture from the air
decreases the relative humidity.
However, it is possible to change the relative humidity of the 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 in
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 in 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 this concept.
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 little 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
imagine the little cup is mostly full.
If we pour this water into the large 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 little cup and the large bucket. Nevertheless, this same amount of water makes
the little cup mostly full and the large bucket mostly empty. We always keep in mind that the little 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 little 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 little cup that
will now be more full. This is
remarkable: there is still the same 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 see that regarding
relative humidity as simply the amount of moisture is a grossly incorrect understanding
of this concept. 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 what if the
thermals are at different temperatures?
The hotter thermal has a greater saturation amount, while the colder
thermal has a lesser saturation amount.
Therefore, 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
little cup that are both half full. The
large bucket holds more water and the small cup holds less water, even though
they are both half full! We always keep
in mind that the little cup is analogous to cold air and the large bucket is
analogous to hot air. If a large bucket
and a little cup are both half full and yet the large bucket holds more water
and the little 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). An an
extreme example of this concept, consider two thermals: one at ninety percent
relative humidity and the other at ten percent relative humidity. Which thermal holds more moisture? We are tempted to conclude that surely it
must be the ninety-percent humid thermal.
In fact, we cannot draw any conclusions about the moistures of the two
thermals without knowing their temperatures.
Again, we imagine a large bucket and a little cup. Suppose the large bucket is only ten-percent
full, while the little cup is ninety-percent full. Nevertheless, the large bucket is so large
that it still hold more water at ten-percent capacity than the little cup at
ninety-percent capacity. We always keep
in mind that the little 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 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 darkness has spent
the entire nighttime cooling the air.
Just before sunrise, the air may have cooled so much that the relative
humidity has increased so much that it has attained one hundred percent
relative humidity. The air has become
saturated with water vapor. At one
hundred percent relative humidity (saturation), water vapor must condense into
liquid water to provide room for further evaporation. 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.
For example, in the early morning, we may see the leaves of trees and
the surface of our cars covered with water as if it had rained overnight. Actually, it became so 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 so cold that the dew point is achieved.
Condensation is the changing
of state from water vapor to liquid water.
The condensing water must liberate heat to its surroundings to condense;
this 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 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 water extracts
the heat needed for that evaporation from our bodies, thus cooling our
bodies. Drying our bodies with a towel
removes this water that would have evaporated; hence, we feel less chilly. This is also why humans and some animals
perspire (sweat). The act of perspiring
(sweating) covers our bodies with water that evaporates; the 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 back into liquid water to provide room for
further evaporation. Whereas
perspiration (sweat) on our skin may evaporate which cools our bodies, some
water vapor 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.
Thus, humid air feels warmer than its actual temperature. We can convert this discomfort into an
effective temperature that is warmer than the actual temperature. This effective 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 is the true temperature,
while the ninety-five degrees is the heat stress index (or heat index for
short). In the wintertime, wind makes
the air feel colder than its actual temperature. We can convert this discomfort into another
effective temperature that is colder than the actual temperature. This effective 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 is the true
temperature, while the twenty-five degrees is the windchill. We may use all of these principles to build a
primitive hygrometer, which is a device that measures the relative humidity of
the air. We build a primitive hygrometer
with two thermometers. One thermometer
is wrapped in a wet cloth; this is called the wet-bulb thermometer. The other thermometer is called the dry-bulb
thermometer. Water will evaporate from
the wet-bulb thermometer. Again,
evaporation is a cooling process. Since
evaporation requires heat, the water will extract heat from the wet-bulb
thermometer, making it a colder temperature than the dry-bulb thermometer. 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 will now apply everything
we have learned 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 cars.
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 the piece of dust or silt or salt. For this reason, this 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. The center of an entire galaxy
is called the galactic nucleus. If the
dew point is achieved and water vapor condenses into microscopic drops of water
around these condensation nuclei, the thermal becomes opaque. Ordinarily, air is transparent, as we know
from daily experience. Every second of
every day of our lives, we effortlessly see through the air around us, since it
is transparent. However, liquid water is
opaque. Actually, small quantities of
liquid water are 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 though 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 and water vapor condenses 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 and thus its water vapor has condensed
into liquid water and thus the thermal has turned from transparent to opaque
(from invisible to visible) is called a cloud.
A cloud forms when a thermal rises, expands adiabatically, cools, and
becomes more humid until the dew point is achieved. This dew point is a specific temperature. Therefore, the thermal 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 with our own eyes, since
clouds often have flat bottoms. This
flat bottom is the lifting condensation level (or condensation level for
short). 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.
Clouds can be categorized
into three broad types: cumulous 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.
This word stratus 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. Note that there 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. 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. 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 sea level. This type of cloud is called fog.
We have already learned
enough meteorology that we can do a fair job predicting weather over the next
several hours using only a barometer.
The rising or the falling of the air pressure is called the barometric
tendency. If the barometric is falling,
then low-pressure, low-density thermals must be rising, expanding
adiabatically, cooling, and becoming more humid. The relative humidity may increase so much
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 so much that liquid water will evaporate back into the
gaseous state. 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 “bad” weather, while a rising
barometric tendency is an indication of “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. Therefore, the wet adiabatic rate of cooling
is always slower than the dry adiabatic rate of cooling. Stated the other way around, the dry
adiabatic rate of cooling is always steeper 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. Its rate of cooling may be sufficiently steep
that it becomes so cold and so dense that it is forced to sink. This is called absolute stability, and “bad”
weather such as clouds or precipitation (rain or snow) will be unlikely. 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. In other
words, thermals are likely to rise. This
is called absolute instability, and “bad” weather such as clouds or
precipitation (rain or snow) will be likely.
It is theoretically 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 sufficiently that it may sink resulting in “good”
weather, but if a thermal reaches the lifting condensation level and becomes a
cloud, its rate of cooling slows. Hence,
it will continue to rise resulting in “bad” weather. This is called conditional instability, and
either “good” weather or “bad” weather may result under these circumstances.
Our arguments lead us to
conclude that what determines “good” weather or “bad” weather is lifting, the
rising of thermals. There are three
mechanisms that could cause lifting: orographic lifting, convergence lifting,
and frontal wedging. Orographic lifting
is when a mountain pushes air upward.
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 the 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 sides that face 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 climate for an additional reason: any moisture that was in the air
probably precipitated out of it on the windward side of the mountain range. With moisture subtracted and on top of this
warming temperatures, we expect an extremely arid (dry) climate on the leeward
side of mountain ranges. These are
called rainshadow deserts. For example, the United States is at the midlatitudes, and the prevailing winds at the midlatitudes blow from the west, as we will explain
shortly. Therefore, the west side of the
Rocky Mountains is the windward side, while the east side of the Rocky
Mountains is the 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
when crowded winds push air upward.
Imagine 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. 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. 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 such a humid
climate. In fact, the whole peninsula is
infested with amphibians and reptiles as a result of this extreme
humidity. Frontal wedging is when one
air mass pushes another air mass upward.
This is the most important type of lifting. Consequently, we will devote a significant
amount of discussion to frontal wedging shortly.
The lightest type of liquid
precipitation is called mist. Heavier
than mist is called 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
the heaviest freezing precipitation is called hail. Hail is quite dangerous; many people have
been killed from falling hail. Snow is
so light because it is composed of individual snowflakes, and a snowflake is
itself composed of mostly air. Since
clouds form at higher elevations where the air temperature is colder,
precipitation almost always begins in the frozen state, such as snow or
sleet. On its way down, 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 that melted into
rain on its way down from the clouds.
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 complicated. 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 a 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. Wind does not always blow in the
directions of these prevailing winds; small variations in pressure may cause
winds to blow in various different directions.
The prevailing winds are the directions the wind generally or usually
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 and be pushed toward the poles, where the
high-pressure, high-density 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, which causes a Coriolis force.
This causes tremendous complications to the simplistic model we have
presented. The low-pressure, low-density
air still rises at the equator and is still pushed toward the poles. However, by the time the air reaches roughly
thirty degrees latitude in each hemisphere, the air has cooled enough to
sink. Once at sea level, the pressure
gradient force pushes this air back to the equator, which completes 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 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 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 is still pushed
toward the equator. However, by the time
the air reaches roughly sixty degrees latitude in each hemisphere, the air has
warmed enough to rise. This air is
pushed toward the poles where it sinks; this completes 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 from 90°N latitude
(the north pole) and 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 from 90°S
latitude (the south pole) and 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, where the air rises and is pushed back to thirty
degrees latitude where it sinks. This
completes 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 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 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 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 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, the two Ferrel
cells (one in each hemisphere) at the midlatitudes,
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 one
circulation cell in each hemisphere. A
beautiful example of this 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, we have
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 around the equator, and the perpetual rain from those 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 to move again. At least the
sailors could drink the perpetual rainwater while stuck at the equatorial low
(the doldrums). At roughly thirty
degrees latitude in each hemisphere, we also have 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 free of
clouds around roughly thirty degrees latitude in each hemisphere. The perpetual lack of rain causes hot deserts
at and near roughly thirty degrees latitude in each hemisphere, such as the
Basin and Range in southwestern United States and northwestern Mexico, 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. 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 winds to move 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 each
hemisphere, we have 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 around roughly sixty degrees
latitude in each hemisphere, and the perpetual rain from those clouds causes
boreal forests (cold forests) at and near roughly 60°N
latitude, such as the Canadian boreal forests and the Russian boreal
forests. Theoretically, there would be
boreal forests (cold forests) at and near roughly 60°S
latitude if there were land at these latitudes.
At the poles, we also have 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 hemisphere. 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 each
hemisphere, we have the subpolar lows at roughly sixty degrees latitude in each
hemisphere, and we have the polar highs at ninety degrees latitude in each
hemisphere. At the lows, we have rising
air, causing perpetual bands of clouds and rain. At the highs, we have sinking air, causing
areas perpetually free of clouds and rain.
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. 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. 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. 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. 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 type of an air mass can
change. For example, if an air mass
forms near the equator, it 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
happen. 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,
if an air mass forms over a continent, it 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 moisture to the air mass. Eventually, we must reclassify it as a
maritime air mass. The reverse can
happen. 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 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 beautiful example of this is lake-effect
snow. The five Great Lakes are between
the United States and Canada, two countries of 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
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 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 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 a
continental air mass before crossing the Great Lakes.
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 this
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 Place 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 theories have further
similarities. Just as there are
different types of tectonic plate boundaries that cause different types of
geological activities, 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 front, as we will explain shortly. A faster-moving cold front can catch up to
and merge with a slower-moving warm front.
This is called an occluded front.
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 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 pointing in two opposite directions.
As a simple example of applying the Bjørgvin
Theory of Meteorology to explain weather patterns, consider the 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.
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 (the First World War or
World War I) roughly one hundred years ago.
This was the most global and most horrific war in human history up until
that time, compelling many people throughout the world to often draw military
analogies. A military front is a
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,
moving 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
named the boundary between two air masses 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 such that the warm air sits on top of the cold
air. In other words, the cold air sits
underneath the warm air. This means that
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, which is more
shallow. Since warm fronts are more
shallow, it takes it a longer time for a warm front to pass. Since cold fronts are more vertical, it takes
a shorter time for a cold front to pass.
This explains why cold fronts pass faster than warm fronts. Along both cold fronts and warm fronts,
rising hot air will expand adiabatically and cool 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 out over a larger area;
consequently, the precipitation along a warm front is often gentle. The common weather associated with a warm
front is gentle precipitation over 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 intense. The
common weather associated with a cold front is intense precipitation over a
brief amount of time, perhaps as short as a couple of minutes, followed by
colder temperatures as compared with the temperatures before the cold front
arrived.
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, every
electrical insulator will conduct electricity if it is subjected to electric
fields of 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 the air exceeds 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. To
summarize, 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 in the outfield of a
baseball field that there is a delay between seeing and hearing a baseball bat
crack a baseball. Some of us notice
while sitting in the infield of a baseball field 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 first see the event,
then we hear the event. This is because
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 five seconds per mile. We can
use this relatively slow propagation of sound to estimate how far away a storm
is occurring. We simply count the number
of seconds after we see lightning until we hear thunder. For every five seconds we count, the storm is
one mile distant. For example, if we see
lightning and count fifteen seconds until we hear thunder, the storm is three
miles away, since every five seconds we counted is one mile. If we count many second after seeing lighting
but never hear thunder, this means that the storm is very far away. Thunder always propagates outwards in all
directions, spreading its 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 even count to 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
part of 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 part of 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 happen in the midwestern
United States.
The Fujita scale (or F-scale)
is a tornado wind-speed 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
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 this, 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 becomes 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 die
quietly. 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 shortly.
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. 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
ocean water onto the continent. This is
called the storm surge. The winds to the
left of the eye push water 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 is from the winds to
the left of the eye. These directions
are reversed in the southern 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 of 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. These six lists have only twenty-one names
each because names beginning with the letters Q, U, X, Y, and Z are not
used. 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 lists are recycled
every six years, but if a hurricane is particularly destructive, then its name
is permanently retired and is forever associated with that hurricane for that
year. A new human name beginning with
the same letter of the alphabet must 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 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 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. The
weakest hurricane/typhoon is called Category 1, stronger is called Category 2,
stronger is called Category 3, stronger is called Category 4, and the strongest
is called Category 5. Keep in mind that
even a Category 1 hurricane/typhoon is stronger and more destructive than a
tropical storm. For example, hurricane
Sandy was a Category 1 hurricane when it attacked and devastated New Jersey in
the year 2012. Even tropical storms,
which are themselves weaker than Category 1 hurricanes, can destroy entire
towns; many people have been killed by tropical storms, themselves weaker than
the weakest hurricanes. We must always
seek shelter during a tropical storm warming, 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, or a few days.
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, 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 all 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 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 example we just mentioned, the daily
temperature range for today would be ten, since eighty minus seventy is
ten. The monthly temperature mean is the
average of all the daily means for that month.
For example, if a 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) and
the coldest monthly mean (almost always January or February) 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, it
is not only essential to 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 along coasts 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 coastal areas
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 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 over the entire history of the Earth, each lasting many millions of
years. The current ice age began roughly
thirty million years ago and still continues to the present day. The Earth will remain in the current ice age
as long as the continent Antarctica remains relatively isolated at the south
pole. 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 living in 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
generally-accepted theory to explain the alternation between glacial periods
and interglacial periods within the current ice age is the Milankovitch
cycles. The Earth’s orbit around the Sun
is 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 planet 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.
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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