This is one of the webpages of Libarid A. Maljian at the Department of Physics at CSLA at NJIT.

 

 

 

New Jersey Institute of Technology

College of Science and Liberal Arts

Department of Physics

The Earth in Space

Phys 203

Spring 2020

Fourth Examination lecture notes

 

 

 

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 (eighty percent) and a fair amount of oxygen gas (twenty percent).  This twenty-percent abundance of oxygen gas is an enormous percentage; 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 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 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 planet 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 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, 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, 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.  In fact, this is this is 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) happens down in 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 the complex carbohydrates as well as sugars, breaking them down into glucose (a simple sugar).  There is a tremendous amount of energy stored in 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 life, we probably mean to use the word “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.  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 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 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 get 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 get 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.  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 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 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 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 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 winter 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 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 is 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 the 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 latitude 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 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 from the Earth, which was the only understanding of primitive humans.  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.  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.  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.  In other words, we cannot see the Sun, making it nighttime.  These extreme northern latitudes and extreme southern latitudes are the only places on Earth were 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 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 is lowest in the sky 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 get warmer, and most substances contract when they get 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 gets warmer, and mercury contracts by quite a noticeable amount as it gets 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 will get 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 get 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 happen 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 water temperature 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 water temperature to cool to minimum temperature sometime in late February.

 

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.  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 shorter 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 longer distance.  In other words, the pressure gradient will be shallow, and we will have light winds.  This is remarkable, since the pressure drop is always a fixed amount: ten millibars.  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 get 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; therefore, 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 get 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; therefore, 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; therefore, 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; therefore, 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 to 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 to 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.  The Coriolis force deflects wind.  Therefore, 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 air from the surrounding high pressure 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 air from the high-pressure thermal outward toward the surrounding low pressure.  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; we will learn later that tornadoes and hurricanes are extreme examples of cyclones.  The weather pattern around a high-pressure thermal is called an anticyclone; will learn later that a beautiful clear day is an extreme example of an anticyclone.  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 the 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 the 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 get warmer; if we extract heat from a gas, we expect it to get cooler.  However, it is possible to change the temperature of a gas without adding or extracting heat.  If a gas expands, it must get cooler even if no heat was extracted.  This is because the expanding gas must push the surrounding gas out of the way which requires work, and work is a form of energy.  The gas extracts this energy from its own internal energy content, and so it gets cooler.  Conversely if a gas contracts, it must get warmer even if no heat was added.  This is because the surrounding gas performs work on the gas compressing it.  This work is added onto the internal energy content of the gas, and so it gets 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 with the gas 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 things.  Most people naively believe that heat and temperature are basically the same thing.  After all, when we add heat to something it often gets warmer, and when we extract heat from something it often gets 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 get warmer under certain circumstances when we have extracted heat!  It is also possible for a gas to get cooler under certain circumstances when we have added heat!  Such examples truly persuade us that heat and temperature are two completely different things.  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 things.  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 isothermal (no temperature change).  Simply because a process is isothermal (no temperature change) does not mean that it is adiabatic (no heat exchanged).  We just discussed two examples of adiabatic processes that are not isothermal.  A gas that expands adiabatically must get cooler; this is not isothermal.  A gas that contracts adiabatically must get warmer; this is not isothermal either.  Also note that a process where there is no pressure change is called an isobaric process.

 

The 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 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 a false understanding.  Firstly, the 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 make 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 we make air 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 we make air 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 since it is able to hold more moisture.  The little cup is analogous to cold air, since cold air has a small saturation amount since 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, it will be mostly empty.  If we take a large bucket that is mostly empty and pour this water into a small cup, it 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.  Always remember 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!  Always remember 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.  Always remember 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 after 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 after 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 make 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.

 

Evaporation is the changing of state from liquid water to water vapor.  The water must absorb 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 make 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.  While evaporation is a cooling process, condensation is a warming process.  Therefore, our bodies cannot cool effectively, and we feel uncomfortable.  Thus, humid air feels warmer than it actually is.  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 it actually is.  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.  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 becomes 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.  The word stratum (a layer of sedimentary rock as we learned earlier in this course) 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, we get a cloud 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 “nice” 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.  As we have argued, we could get either “good” weather or “bad” weather in these circumstances.

 

Our arguments lead us to conclude that what determines “good” weather or “bad” weather is whether or not thermals will rise.  The rising of thermals is called lifting, and there are three types: orographic lifting, convergence lifting, and frontal wedging.  Orographic lifting is when a mountain pushes air upward.  When winds encounter a mountain, the air has no choice but to go up over the mountain.  As the air rises, it expands adiabatically, cools, and becomes more humid.  If the dew point is achieved, we get clouds and possibly precipitation.  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 another 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 a midlatitude, and the prevailing winds at the midlatitudes blow from the west.  We will explain why this is 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 they have no choice but to push each other upward.  As the air rises, it expands adiabatically, cools, and becomes more humid.  If the dew point is achieved, we get clouds and possibly precipitation.  Therefore, we expect islands and peninsulas to have humid climates.  Actually, we expect the climate to be extremely humid, since the winds 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 push each other upward.  The air rises, expands adiabatically, cools, and becomes even more humid.  The dew point is achieved, and we get clouds and rain.  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 made of individual snowflakes, and snowflakes are 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.

 

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 result 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) blows from the southeast; these are called the southeast trade winds since we always name wind based on the direction it is blowing from.  The word trade 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 result 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 blows 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 result 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 near equator, the two Ferrel cells at the midlatitudes, and the two polar cells near the poles.  If the Earth rotated faster, the Coriolis force would be stronger causing more prevailing winds and more circulation cells in each hemisphere.  If the Earth rotated slower, the Coriolis force would be weaker 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 a 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 get stuck, since there are no winds to push the ship.  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 get moving 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 get stuck, since there are no winds to push the ship.  There would also be no rain for the sailors to drink.  Therefore, not only would sailors pray that their ship happens to drift slight to the north or slightly to the south to catch winds to get moving 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 continent North America.  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.

 

According to the fundamental 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 around by the prevailing winds, and much meteorological activity (commonly known as weather) happens at the boundary between two air masses which is called a front.  This fundamental theory of meteorology is remarkably similar to the fundamental theory of geology (the Theory of Plate Tectonics).  As we discussed earlier in the course, the fundamental theory of geology (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 around by convection cells in the asthenosphere (underneath the lithosphere), and much geological activity happens 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; we will explain why this is the case 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 fundamental theory of meteorology to understand 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) is 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.  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 word front is borrowed from military terminology.  The fundamental theory of meteorology was formulated during and shortly after the Great War (the First World War) roughly one hundred years ago.  This was the most global and most horrific war in human history up until that time, causing 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 a stationary military front, the textbook example being the western front of the Great War.  The western front remained stationary for most of the years of the 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 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.  Meteorologists 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 move 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 exceed 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 sound.  This explosive sound is 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 later than 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 the slow propagation of sound to estimate how far away a storm is.  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 North America and is near the north pole, while cT air masses (continental tropical air masses) form over Mexico, since Mexico is part of North American and is 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.  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 in a moment.  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, making most of the destruction to the left of the eye 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.  We will learn later in this course that flooding is the most common and 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 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 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 2013 should have been named Dean, but hurricane Dean was so destructive in 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 2005 was tropical storm Alpha, the twenty-third was tropical storm Beta (later promoted to hurricane Beta), the twenty-fourth was tropical storm Gamma, the twenty-fifth was tropical storm Delta, the twenty-sixth was 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 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.  Always seek shelter during a tropical storm warming, and certainly always seek shelter during a hurricane/typhoon warning, regardless of its Saffir-Simpson category number.

 

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 Antarctica remains relatively isolated at the south pole.  The southern icecap covers Antarctica, and the northern icecap covers 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 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 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.

 

 

 

Links

 

Libarid A. Maljian homepage at the Department of Physics at CSLA at NJIT

Libarid A. Maljian profile at the Department of Physics at CSLA at NJIT

Department of Physics at CSLA at NJIT

College of Science and Liberal Arts at NJIT

New Jersey Institute of Technology

 

 

 

This webpage was most recently modified on Wednesday, the fifteenth day of April, anno Domini MMXX, at 02:45 ante meridiem EDT.