Download Chapter 1 The Nature and Structure of the Atmosphere

The Nature and Structure of the Atmosphere
Chapter 1 The Nature and Structure of the
Atmosphere
1.1 Introduction
We will define Meteorology as the physics of the atmosphere. If you look at
the word carefully, this definition may seem a little strange, because most
words that end in the suffix “logy” represent the study of that part of the
word that comes first. That is, biology is the study of bio or life; while
sociology is the study of society; geology is the study of the geo or the earth,
and so forth. So the first attempt at understanding the word meteorology
would lead you to think that meteorology is the study of meteors. Where did
this word come from? The word meteorology comes from the Greek word
meteorologica, meaning the discourse or study of the things that are up above.
The great Greek philosopher Aristotle (384-322 BC) wrote a treatise, in about
340 BC, entitled “Meteorologica” in which he tried to describe everything of a
physical nature in the sky. The word meteor meant anything that came from
the sky. It included not only meteoroids or meteorites that are masses of stone
or iron that fall from the sky; but it also included aerial meteors, which are
winds, and hurricanes; aqueous meteors, which are rain, snow, hail, etc.; and
luminous meteors, which are auroras, rainbows, etc. Hence, meteorology was
the study of all the things that occurred in the atmosphere.
Many of Aristotle’s explanations of the different phenomena that occur
in the atmosphere are not acceptable by today’s standards. What is extremely
important, however, is that Aristotle’s treatise was an attempt to describe the
world in terms of rational explanations, rather than in terms of the mythology
of the day. The ancient Greek mythology included numerous gods who were
supposed to control all things, celestial or terrestrial. There was Zeus, the
ruler of the heavens, who controlled the clouds, rain and thunder, who would
throw thunderbolts at the earth if something displeased him. Helios was the
sun god, and Aeolus was the god who controlled the winds. Poseidon was
the god of the sea. Hades was the god of the underworld.
Aristotle was one of a group of philosophers, figure 1.1, who tried to
seek a rational explanation of the world about them, an explanation of the
nature of the world without recourse to magic, myths, or revelation. They
questioned the cause of the regularity in the motion of the heavenly bodies.
What makes the sun rise, move across the sky, and then set? What makes the
stars and moon move in the night sky? What is the earth made of? What is
man? Philosophy was the search for knowledge or wisdom (philos in Greek
means “love of” and sophos means “wisdom”). The study of the natural world
itself was called natural philosophy. The Greek word for “natural” is
physikos. Therefore, the name physics came to mean the study of the physical
or natural world. Hence, the study of the atmosphere will be called
Meteorology, the physics of the atmosphere.
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The Nature and Structure of the Atmosphere
Figure 1.1 The school of Athens showing the ancient Greek philosophers.
Let us start our study by looking at the entire earth itself. The study of
the earth can be separated into four areas or spheres of interest. They are:
1. The Lithosphere. The lithosphere is the study of the solid earth. We
live on the outer skin of the earth, which is called the crust. The crust is very
thin, averaging only about 20 km in thickness. To see how small this really
is, let us consider what percentage the crust is of the entire earth. The radius
of the earth is 6371 km. If we divide the 20 km of the earth’s crust by the
radius of the earth itself, and then multiply by 100% we get
20 km  100 % = 0.314%
6371 km
Hence, the thickness of the crust is only about 0.31 % of the radius of the
entire earth. Although the earth may seem rather large, the crust that we
live on is indeed very small. A complete study of the solid earth is found in
the science of Geology.
2. The Hydrosphere. The hydrosphere is the study of all the water on
the earth. Since the oceans make up about 97% of the earth’s water, it is
essentially a study of the oceans. A complete study of the hydrosphere is found
in the science of Oceanography.
3. The Atmosphere. The atmosphere is the study of all the air
around the earth. It constitutes the science of Meteorology, the physics of
the atmosphere.
4. The Biosphere. The Biosphere is the study of the life forms on
the earth. It constitutes the science of Biology.
These spheres do not exist alone. The study of weather focuses on the
atmosphere but there is a continuous interaction with the other spheres.
We have defined Meteorology as the physics of the atmosphere. We use
the methods of physics to interpret and explain the many processes that
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The Nature and Structure of the Atmosphere
occur in the atmosphere. Weather, on the other hand, is defined as the day to
day state of the atmosphere at a given time and place and pertains to short
term changes (the present time to about 72 hours) in conditions of :
1. Heat (Heat supplies the energy to drive the atmosphere.)
2. Moisture (Moisture supplies the water for the formation of
clouds and precipitation, and also supplies energy to be transferred
through the latent heat of vaporization.)
3. Air pressure (The differences in air pressure in the atmosphere
cause the motion of air or winds in the atmosphere.)
In the study of meteorology we want to know what causes the
variations in (1) heat exchange, (2) moisture exchange and, (3) air pressure,
from the time t1 to the time t2 and from the position x1 to the position x2. But
remember, the primary source of all energy for the interactions that occur in
the atmosphere is the sun. In particular, weather results from processes that
attempt to equalize the differences in the distribution of the net radiant energy
from the sun.
1.2 The Nature of the Atmosphere
The atmosphere is a deep blanket of gases and suspended liquids and
solids that entirely envelops the earth. Compared to the radius of the earth
however, the atmosphere is a very thin layer of air. The radius of the earth is
6371 km while the height of the atmosphere above the surface of the earth is
very small. Half the atmosphere lies below a height of 6 km (18,000 ft), while
99% of the atmosphere lies below a height of 30 km (20 miles) above the
surface of the earth. Usually when we draw a picture of the atmosphere above
the surface of the earth we usually show a very thick atmosphere, figure 1.2.
We see now that although this is convenient for showing the atmosphere, it is
certainly not correct. In fact if we divide the 30 km that contains 99% of the
earth’s atmosphere by the radius of the earth itself we get
30 km = 0.00471
6371 km
or the height of the atmosphere is only about 0.47 % of the radius of the
earth. Hence the layer of the atmosphere is indeed very small. If we tried to
draw the atmosphere to scale in figure 1.2, the atmosphere would be about as
thick as the circular line itself, and you wouldn’t be able to see the
atmosphere at all. Hence, in all our drawings we will show the atmosphere as
being a thick layer, but remember its real size is really very small compared
to the size of the earth itself.
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The Nature and Structure of the Atmosphere
Figure 1.2 Diagram of the earth and its atmosphere.
1.3 The Composition of the Atmosphere
Air is a mixture of several gases, each of which acts independently of
the others. The constituents of dry air and their abundance are shown in
table 1.1.
Table 1.1 Constituents of Dry Air
Constituent
Percentage by Volume
Nitrogen N2
Oxygen O2
Argon A
Carbon Dioxide CO2
Neon Ne
Helium He
Methane Ch4
Krypton Kr
Hydrogen H
Plus trace quantities of
Nitrous Oxide N2O,
Xenon, Ozone O3, Radon,
and Dust particles
78.084 %
20.946 %
0.934%
0.035%
0.00182
0.000524
0.00015
0.000114
0.00005
When the air also contains water vapor, the air is called moist air.
Depending on the pressure and temperature of the air, the air can hold up to
about 4% of water in it. The amount of water vapor can vary greatly from
place to place and time to time.
The characteristics and effects of the different gasses and other
constituents in the atmosphere are:
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The Nature and Structure of the Atmosphere
1) Nitrogen - The gas with the greatest abundance in the atmosphere
is nitrogen, and yet it has a very small effect upon the weather on the earth,
because it is a very poor absorber of the incoming radiation from the sun. The
atmosphere contains about 78% of nitrogen. Yet it is extremely important in
the atmosphere because it is responsible for controlling combustion. Recall
from table 1.1, that the atmosphere contains almost 21% of oxygen gas. What
many people do not realize is that oxygen gas supports combustion. If there
were no nitrogen gas in the earth’s atmosphere to control combustion, and
the rest of the atmosphere contained oxygen, then if someone were to light a
match, the entire atmosphere, and everything here on earth would go up in a
puff of smoke. In the early days of the Apollo space program, both the
Command Module and the Lunar Module were designed to contain an
environment of 100% oxygen. The reason for this was the pressure within the
space capsules was to be kept relatively low in order to help prevent leaks
from the capsule in the vacuum of space. However, nitrogen gas is known to
cause the bends in the human body when it goes from low pressure back to
higher pressures. To prevent this possibility from occurring, both the
Command and Lunar Module would have an environment of 100% oxygen.
Although it was known that a 100% oxygen environment was dangerous,
because of the possibility of a spark creating a fire, it was assumed that the
probability of it occurring was lower than the probability of the bends
occurring to the astronauts. As I‘m sure you are aware, this was a mistake. In
the simulated launch ground tests of Apollo 1 in 1967, an electrical spark
ignited the 100% oxygen environment in the Command Module killing
astronauts Gus Grissom, Roger Chaffee, and Ed White. Hence, having the
large percentage of nitrogen in our atmosphere protects all life on earth.
2) Oxygen - The oxygen gas in our atmosphere absorbs some
ultraviolet radiation from the sun. The greatest contribution of oxygen to our
atmosphere is that it supports life. Human beings and many other creatures
on this earth breathe oxygen to stay alive. Without it we would all soon cease
to exist.
3) Argon - Argon is an inert gas that does not interact with any
chemicals on the earth or in the atmosphere. When one chemical reacts with
another chemical, it is usually done by an exchange of an electron from the
shell of one chemical element to the shell of another. The outer shell of Argon
is filled with all the electrons it can hold, and therefore does not react with
anything.
4) Carbon Dioxide - Carbon dioxide is a gas in our atmosphere that
absorbs a part of the long wave radiation from the earth and as such is
involved in the Greenhouse effect, the name for the heating of the
atmosphere. Although the amount of carbon dioxide in the atmosphere is
relatively small, its percentage has been growing steadily. There is a fear
that too much carbon dioxide in the atmosphere will lead to excessive heating
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The Nature and Structure of the Atmosphere
of the atmosphere, which will lead to Global Warming. We will discuss Global
Warming in detail in the next chapter.
5) Ozone - Ozone gas, found in a layer in a portion of our atmosphere
called the stratosphere, absorbs a very large quantity of the ultraviolet
radiation from the sun. Ultraviolet light is rather energetic and can cause
skin cancer in humans. The ozone layer absorbs this ultraviolet light
protecting us from skin cancer. A group of man made chemicals, referred to
as Chlorofluorocarbons or CFC’s for short, can destroy the ozone layer and is
a potential hazard. CFC’s, sometimes called Freons, are used in airconditioning and refrigeration systems, the production of some plastics, and
the propellant in some aerosol cans. These chemicals do not react with other
chemicals in the lower atmosphere. They eventually end up rising up to the
stratosphere, where they react with the ozone, thereby depleting it. If the
ozone layer were to be destroyed, all that ultraviolet light from the sun would
reach the surface of the earth and do a great deal of damage to humans,
crops, and ecosystems. The amount of Ozone in the atmosphere is very small,
approximately 0.000004%.
6) Nitrous Oxide - Although the amount of nitrous oxide in the
atmosphere is quite small, 0.00003%, its effects are very significant. Through
the burning of large quantities of fossil fuels, nitrous oxide gas is given off as
a byproduct. This nitrous oxide combines with the water droplets in clouds to
form nitric acid, a very powerful acid. When the rain starts to fall from these
clouds it is called acid rain. The effect has been very noticeable in regions
downwind from some industrial cities. As an example, pollutants from the
Chicago area are moved through the atmosphere to some of the large lakes in
upstate New York. The rain from these polluted clouds has been so acidic,
that the lakes have become very acidic, killing off the fish that used to live in
these lakes. This is equivalent to putting nitric acid into your fish tank in
your home. The fish will die off very rapidly. The acid rain has also been
responsible for burning trees, shrubs, and grass in the affected areas. Nitrous
oxide is also a Greenhouse gas, absorbing the infrared radiation from the
earth. Larger quantities of nitrous oxide in the atmosphere could also
contribute to Global Warming.
7) Water vapor H2O - Water vapor is one of the most important
constituents of the air. It causes fog, clouds, haze, ice crystals, and
precipitation. It also absorbs, reflects, and scatters certain wavelengths of
solar and terrestrial radiation. The absorption of terrestrial radiation by
water vapor is one of the most important processes for the heating of
the atmosphere. Because water vapor absorbs radiation from the earth, in
regions on the earth where there is a large water vapor content in the air, the
air will be warmer than in regions where there is only a small quantity of
water vapor in the air. Deserts are very cold at night because as the earth
radiates energy away there is no water vapor in the air to absorb it.
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The Nature and Structure of the Atmosphere
8) Dust Particles - Dust particles in the atmosphere absorb, reflect,
and scatters incoming solar radiation. The beautiful red colors at sunrise and
sunset are caused by scattering of light from dust particles and molecules in
the air. Also an extremely important effect of dust particles, is that the
condensation of water vapor begins on dust particles in the atmosphere.
Without these particles there would be no clouds or rain. Such dust particles
are called hygroscopic nuclei.
9) Methane, Nitrous Oxide N2O, and Chlorofluorocarbons
(CFC’s) are also considered greenhouse gases and contribute to the heating
of the atmosphere.
1.4 The Vertical Divisions of the Atmosphere
The atmosphere is divided into four divisions based upon the
distribution of temperature in the vertical. They are:
1. The Troposphere
2. The Stratosphere
3. The Mesosphere
4. The Thermosphere
This distribution is shown in figure 1.2.
Figure 1.2 Vertical distribution of temperature in the atmosphere.
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The Nature and Structure of the Atmosphere
1) The Troposphere. The troposphere (where the air turns over) is the
lowest portion of the atmosphere. Three-quarters of the total atmospheric
mass is contained in the troposphere. It is the area of clouds, storms, and
convective motion. It is sometimes called the weather sphere. The
characteristic of the troposphere is that there is a uniform decrease in
temperature with altitude until the temperature of −50 0C or −60 0C is
reached. The change in temperature with altitude is called a lapse
rate. This lapse rate is called the normal lapse rate and it is equal to 6.5 0C
per kilometer. The boundary between the troposphere and the stratosphere is
called the tropopause. The tropopause is not at a constant height but varies
from a height of 8 km at the poles to 16 km at the equator (about 20,000 ft to
65,000 ft). It averages about 12 km. Hence the lowest temperature in the
troposphere is found at the equator not the poles because the height of the
troposphere varies with latitude, and the air keeps cooling through the
additional height.
2) The Stratosphere. The region of the atmosphere called the stratosphere,
begins where the temperature stops decreasing with altitude and becomes
constant. It is in the stratosphere that ozone is located. Recall that ozone
absorbs ultraviolet radiation from the sun. This absorbed energy warms the
air at this level and not only does the temperature no longer decrease with
altitude but instead it actually increases as can be seen in figure 1.2. The
stratosphere extends from about 10 km up to about 47 km above the surface
of the earth.
3) The Mesosphere. The mesosphere begins at about 47 km where the
temperature no longer increases with height because there is no more ozone
above this level to absorb any ultraviolet radiation from the sun. The
temperature actually remains constant for a short distance and then starts to
decrease with altitude up to a level of about 80 km.
4) The Thermosphere. The Thermosphere begins at about 80 km above the
surface of the earth. The temperature of the air stops decreasing with
altitude, becomes constant, and then starts to increase with height. This
increased temperature is caused by the absorption of short wave radiation by
atoms of oxygen (O) and nitrogen (N) which are located at that level. This
higher temperature is however misleading. Temperature, as we will see later,
is a measure of the mean kinetic energy of the molecules making up the
substance, and as such is proportional to the square of the velocity of the
atoms or molecules. As you sit in your chair, you are constantly bombarded
by the molecules in the air. As each molecule hits you, it imparts some of that
energy to you and you thus feel warm.
Because the velocity of the atoms at this height is very large, the
temperature is defined to also be very high. However, there are very few
molecules at this height, and therefore there are very few molecules to hit
you or any other object that might be at that height, and therefore very little
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The Nature and Structure of the Atmosphere
energy will be transferred to you. Even though the temperature might be
defined to be high you would not feel warm there.
1.5 A Quick Synopsis of Meteorology
One of the best ways to learn about meteorology is to watch the daily
weather reports on television. At first you may not understand all the lines
on the weather map and the different concepts that the meteorologist is
talking about, but keep watching. As you go through this course we will
explain all these concepts to you. We will now give a very brief synopsis of
what you will see on a weather map and in so doing we will give a brief
summary of this entire course.
Have you ever wondered, while watching the weather forecast on your
local TV station, what all those lines and arrows were on those maps? It
looked something like figure 1.3. If we were to look at the television screen
more closely we would see a map of the United States. At every weather
station throughout the United States, the atmospheric pressure is measured
and recorded on a weather map. On that map, a series of lines, connecting
those pressures that are the same, are drawn.
H
Rain, rain
go away ...
c
P
c
P
L
c
P
c
c
P
P
m
m
T
T
TV
News
“Tomorrow we will have one arrow sticking into another arrow.”
Figure 1.3 Your TV Weather man.
These lines are called isobars and can be seen in figure 1.4. An isobar
is a line along which the pressure is constant. An isobar is analogous to a
contour line that is drawn on a topographical map to indicate a certain height
above mean sea level. As an example, consider the mountain and valley
shown in figure 1.5(a). A series of contour lines are drawn around the
mountain at constant heights above sea level. The first line is drawn at a
height H = 200 m above sea level. Everywhere on this line the height is
exactly 200 m above sea level. The next contour line is drawn at H = 400 m.
Everywhere on this line the height is exactly 400 m above sea level. Between
the 200 m contour and the 400 m contour line the height varies between 200
m and 400 m. The contour line for 600 m is also drawn in the figure. The very
top of the mountain is greater than the 600 m and is the highest point of
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The Nature and Structure of the Atmosphere
H
cP
cP
L
cP
cP
cP
mT
mT
Figure 1.4 A Surface weather map showing isobars, pressure systems, and
fronts.
600 m
400 m
200 m
sea level
-200 m
-400 m
-600 m
(a)
Mountain
Valley
600 m
-200 m
400 m
-400 m
200 m
-600 m
(b)
Figure 1.5 Contour lines on a topographical map.
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The Nature and Structure of the Atmosphere
the mountain. The contour lines showing the valley are drawn at −200 m,
−400 m, and −600 m. The −200 m contour line shows that every point on this
line is 200 m below sea level. The bottom of the valley is the lowest point in
the valley. If we were to look down on the mountain and valley from above,
we would see a series of concentric circles representing the contour lines as
they are shown in figure 1.5(b). (On a real mountain and valley the contours
would probably not be true circles.)
The isobars are to a weather map as contour lines are to a
topographical map. The isobars represent the pressure of the atmosphere. By
drawing the isobars, a picture of the pressure field is obtained. Normal
atmospheric pressure is 29.92 in. of Hg or 1013.25 mb. But remember that
normal is an average of abnormals. At any given time, the pressure in the
atmosphere varies slightly from this normal value. If the atmospheric
pressure is greater than normal at your location, then you are in a region of
high pressure. If, on the other hand, the atmospheric pressure is less than
normal at your location, then you are in a region of low pressure. The isobars
indicating high and low pressure are shown in figure 1.6(a). The high-
1025 mb
1010 mb
995 mb
1020 mb
1000 mb
1015 mb
1005 mb
(a)
Pressure
Mountain 1025 mb
1020 mb
1015 mb
sea level
1010 mb
1005 mb
1000 mb
995 mb
Pressure
Valley
(b)
Figure 1.6 High and low atmospheric pressure.
pressure region can be visualized as a mountain and the low-pressure region
as a valley in figure 1.6(b). Air in the high-pressure region flows down the
pressure mountain into the low-pressure valley, just as a ball would roll down
a real mountain side into the valley below. This flow of air is called wind.
Hence, air always flows out of a high-pressure area into a low-pressure area.
The force on a ball rolling down the mountain is the component, acting down
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The Nature and Structure of the Atmosphere
the mountain, of the gravitational force on the ball. The force on a parcel of
air is caused by the difference in pressure between the higher pressure and
the lower pressure. This force is called the pressure gradient force (PGF) per
unit mass, and it is directed from the high-pressure area to the low-pressure
area. It is effectively the slope of the pressure mountain-valley. A large
pressure gradient, corresponding to a steep slope, causes large winds,
whereas a small pressure gradient, corresponding to a shallow slope, causes
very light winds.
If the earth were not rotating, the air would flow perpendicular to the
isobars. However, the earth does rotate, and the rotation of the earth causes
air to be deflected to the right of its original path. The deflection of air to the
right of its path in the northern hemisphere is called the Coriolis effect.
The Coriolis effect is caused by the rotation of the earth and can best
be described by an example. If the earth were not rotating and a projectile,
aimed at a point on the equator, were fired from the north pole, its path
through space would be in a fixed vertical plane that has the north pole as
the starting point of the trajectory and the point on the equator as the ending
point of the trajectory, figure 1.7(a).
N
N
Equator
Equator
S
S
(a) Nonrotating earth.
(b) Rotating Earth
Figure 1.7 The Coriolis force.
However, the earth rotates, and if a projectile, aimed at a point on the
equator, were fired from the north pole, its path through space would be in a
fixed vertical plane that has the north pole as the starting point of the
trajectory and the point on the equator as the ending point of the trajectory
at the moment that the projectile is fired. By the time that the projectile
would arrive at the end point of its trajectory, that point would no longer be
there, because while the projectile is in motion, the earth is rotating, and the
point will have rotated away from the initial position it was in when the
projectile was fired. A person fixed to the rotating earth would see the
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The Nature and Structure of the Atmosphere
projectile veer away to the right of its initial path, figure 1.7(b), and would
assume that a force were acting on the projectile toward the right of its
trajectory. This fictitious force is called the Coriolis Force.
This effect on a projectile is the same effect that would occur to a
parcel of air moving south from the north pole. A person fixed to the rotating
earth would see the parcel of air initially heading south but then it would
appear to veer away to the right of its initial path. Again this apparently
strange effect, caused by an observation on the rotating earth, is taken into
account by assuming that there is a fictitious force, called the Coriolis force
(CF) that acts to the right of the path of a parcel of air in its motion through
the atmosphere.
The equation for the Coriolis force is
CF = 2vΩ sin φ
(1.1)
where CF is the Coriolis force per unit mass of air, v is the speed of the wind,
Ω is the angular velocity of the earth, and φ is the latitude angle. Thus, the
Coriolis force depends on the speed of the air (the greater the speed the
greater the force) and the latitude angle φ. At the equator, φ = 0 and sin φ = 0,
and hence there is no force of deflection at the equator. For φ = 900, sin φ = 1,
hence the maximum force and deflection occur at the pole.
Let us describe the motion of the air as it moves toward the lowpressure area. The air starts on its motion at the point A, figure 1.8(a), along
a path that is
C.F
L
L
A P.G.F
B
P.G.F
v
P.G.F.
f
C.F
v
C.F
(b) Friction
(a) No friction
Figure 1.8 A low-pressure area.
perpendicular to the isobars. But the air is deflected to the right of its path by
the Coriolis force, and ends up at the position B. At B, the pressure gradient
force is still acting toward the center of the low-pressure area, while the
Coriolis force, acting to the right of the path, is opposite to the pressure
gradient force. An approximate balance 1 exists between the two forces and
A more detailed analysis by Newton’s second law of motion would give
a = F = PGF + CF
m
1
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The Nature and Structure of the Atmosphere
the air parcel now moves parallel to the isobars. Notice that the air moves
counterclockwise in a low-pressure area.
As the air moves over the ground, there is a frictional force f that acts
on the air, is directed opposite to the direction of motion of the air, and is
responsible for the slowing down of the air. This is shown in figure 1.8(b).
But, as seen in equation 1.1, the Coriolis force is a function of the wind speed.
If the wind speed decreases because of friction, the Coriolis force also
decreases. Hence, there is no longer the balance between the pressure
gradient force and the Coriolis force and the air parcel now moves toward the
low-pressure area. The combined result of the pressure gradient force, the
Coriolis force, and the frictional force, causes the air to spiral into the lowpressure area, as seen in figure 1.8(b).
The result of the above analysis shows that air spirals counterclockwise
into a low-pressure area at the surface of the earth. But where does all this air
go? It must go somewhere. The only place for it to go is upward into the
atmosphere. Hence, there is vertical motion upward in a low-pressure area.
Now the pressure of the air in the atmosphere decreases with altitude.
Hence, when the air rises in the low-pressure area it finds itself in a region of
still lower pressure aloft. Therefore, the rising air from the surface expands
into the lower pressure aloft. For a gas to expand the gas must do work and
expend energy. Since there is no heat added to, or taken away from this
rising air, the air is said to be expanding adiabatically. The work done in the
expansion of the air causes a decrease in the internal energy of the air.
Hence, the rising air cools as it expands because the energy necessary for the
gas to expand comes from the internal energy of the air itself. Hence the
temperature of the air decreases as the air expands and the rising air cools.
Since the air parcel is moving in a circle of radius r, with a velocity v, the acceleration is the
centripetal acceleration given by v2/r. Hence Newton’s second law should be written as
v2 = PGF + CF
r
But in very large scale motion, such as over a continent, v2/r ≈ 0.1 10−3 m/s2, while the PGF
≈ 1.1  10−3 m/s2. Thus the centripetal acceleration is about 1/10 of the acceleration caused
by the pressure gradient force, and in this simplified analysis is neglected. The second law
then becomes
0 = PGF + CF
or
PGF = −CF
Hence the force on the air parcel is balanced between the pressure gradient force and the
Coriolis force. The wind that results from the balance between the PGF and the CF is called
the geostrophic wind. For a more accurate analysis and especially in smaller sized pressure
systems such as hurricanes and tornadoes this assumption cannot be made and the
centripetal acceleration must be taken into account.
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The Nature and Structure of the Atmosphere
The amount of water vapor in the air is called humidity. The maximum
amount of water vapor that the air can hold is temperature dependent. That
is, at high temperatures the air can hold a large quantity of water vapor,
whereas at low temperatures it can only hold a much smaller quantity. If the
rising air cools down far enough it reaches the point where the air has all the
water vapor it can hold. At this point the air is said to be saturated and the
relative humidity of the air is 100%. If the air continues to rise and cool, it
cannot hold all this water vapor. Hence, some of the water vapor condenses to
tiny drops of water. These drops of water seem to float in the air. (They are
held up by the rising air currents.) The aggregate of all these tiny drops of
water suspended in the air is called a cloud. Hence, clouds are formed when
the rising air is cooled to the condensation point. If the rising and cooling
continue, more and more water vapor condenses until the water drops get so
large that they fall and the falling drops are called rain. In summary,
associated with a low-pressure area in the atmosphere is rising air. The
cooling of this adiabatically expanding air causes the formation of clouds,
precipitation, and general bad weather. Thus, when the weatherman says
that low pressure is moving into your area, as a general rule, you can expect
bad weather.
Everything we said about the low-pressure area is reversed for a highpressure area. The pressure gradient force points away from the highpressure area. As the air starts out of the high-pressure area at the point A,
figure 1.9(a), it is moving along a path that is perpendicular to the isobars.
H
H
P.G.F
A
C.F C.F
v
C.F
B
v
f
P.G.F
P.G.F
(a) No friction
(b) Friction
Figure 1.9 A high-pressure area.
The Coriolis force now acts on the air and deflects it to the right of its path.
By the time the air reaches the point B, the pressure gradient force is
approximately balanced by the Coriolis force, 2 and the air moves parallel to
the isobars. Thus, the air flows clockwise around the high-pressure area. The
frictional force slows down the air and causes the Coriolis force to decrease in
The same approximation for the balance between the PGF and the CF used in the analysis
of the low-pressure area is also made for the high-pressure area.
2
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The Nature and Structure of the Atmosphere
size. The pressure gradient force is now greater than the Coriolis force, and
the air starts to spiral out of the high-pressure area, figure 1.9(b).
From what we have just seen, air spirals out of a high pressure area at
the surface of the earth. But if all the air that was in the high-pressure area
spirals out, what is left within the high-pressure area? If the air is not
replenished, the area would become a vacuum. But this is impossible.
Therefore, air must come from somewhere to replenish the air spiraling out of
the high pressure area at the surface. The only place that it can come from is
from the air aloft. That is, air aloft moves downward into the high-pressure
area at the surface. Thus, there is vertical motion downward in a highpressure area.
As the air aloft descends, it finds itself in a region of still higher
pressure and is compressed adiabatically. Thus, work is done on the gas by
the atmosphere and this increase in energy shows up as an increase in the
internal energy of the air, and hence an increase in the temperature of the
descending air. Thus, the air warms up adiabatically as it descends. Because
warmer air can hold more water vapor than colder air, the water droplets that
made up the clouds evaporate into the air. As more and more air descends,
more and more water droplets evaporate into the air until any clouds that
were present have evaporated, leaving clear skies. Hence, high-pressure areas
are associated with clear skies and, in general, good weather. So when the
weatherman tells you that high pressure is moving into your area, you can
usually expect good weather.
Now when you look at your TV weather map, look for the low- and
high-pressure areas. If the low-pressure area is moving into your region, you
can expect clouds and deteriorating weather. If the high-pressure area is
moving into your region, you can expect improving weather with clear skies.
Those other lines on the weather map are called fronts. A front is a
boundary between two different air masses. An air mass is a large mass of air
having uniform properties of temperature and moisture throughout the
horizontal. Air sitting over the vast regions of Canada has the characteristic
of being cold and dry. This air mass is called a continental polar air mass and
is designated as a cP air mass. Air sitting over the southern ocean areas and
the Gulf of Mexico has the characteristic of being hot and humid. This air
mass is called a maritime tropical, mT, air mass. These two air masses
interact at what is called the polar front. Much of your weather is associated
with this polar front. If the continental polar air mass is moving forward, the
polar front is called a cold front. On a weather map the cold front is shown
either as a blue line or, if the presentation is in black and white, a black line
with little triangles on its leading edge showing the direction of motion. If the
continental polar air mass is retreating northward, the polar front is called a
warm front. On a weather map the warm front is shown either as a red line
or, if the presentation is in black and white, a black line with little
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The Nature and Structure of the Atmosphere
semicircles on its edge showing the direction in which the front is retreating.
The center of the polar front is embedded in the low-pressure area.
With all this background, let us now analyze the weather map of figure
1.4. Notice that there is a large low-pressure area over the eastern half of the
United States. In general, the poorer weather will be found in this region. A
high-pressure area is found across the western half of the United States. In
general, good weather will be found in this region. The polar front can also be
seen in figure 1.4. The cold front is the boundary between the cold continental
polar air that came out of Canada and the warm moist maritime tropical air
that has moved up from the gulf. The arrows on the map indicate the velocity
of the air. The cold dry cP air, being heavier than the warm tropical mT air,
pushes underneath the mT air, driving it upward. The moisture in the rising
tropical air condenses and forms a narrow band of clouds along the length of
the cold front. The precipitation usually associated with the cold front is
showery.
The warm front is the boundary of the retreating cool air and the
advancing warm moist air. The mT air, being lighter than the retreating cP
air, rises above the colder air. The sloping front of the retreating air is much
shallower than the slope of the advancing cold front. Therefore, the mT air
rises over a very large region and gives a very vast region of clouds and
precipitation. Thus the weather associated with a warm front is usually more
extensive than the weather associated with a cold front.
Your weather depends on where you are with respect to the frontal
systems. If you are north of the warm front in figure 1.4, such as in Illinois,
Ohio, or Pennsylvania, the temperatures will be cool, the winds will be from
the southeast, the sky will be cloudy, and you will be getting precipitation. If
you are south of the warm front and in advance of the cold front, such as in
Alabama, Georgia, South Carolina, and Florida, the temperature will be
warm, the humidity high, winds will be from the southwest and you will
usually have nice weather. If the cold front has already passed you by, such
as in Kansas, Oklahoma, Texas, and Arkansas, the skies will be clear or at
least clearing, the temperature will be cold, the humidity will be low, the
winds will be from the northwest, and in general you will have good weather.
All the highs, lows, and fronts, move across the United States from the
west toward the east. So the weather that you get today will change as these
weather systems move toward or away from you.
The Language of Meteorology
Meteorology - is the physics of the atmosphere. An analysis of
the causes of weather
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The Nature and Structure of the Atmosphere
Weather - is defined as the day to day state of the atmosphere at a
given time and place and pertains to short term changes in conditions of :
Heat, Moisture, and Air pressure.
The study of the earth can be separated into four areas or spheres of
interest. They are:
1. The Lithosphere. The lithosphere is the study of the solid earth. A
complete study of the solid earth is found in the science of Geology.
2. The Hydrosphere. The hydrosphere is the study of all the water on
the earth. A complete study of the hydrosphere is found in the science of
Oceanography.
3. The Atmosphere. The atmosphere is the study of all the air around
the earth. It constitutes the science of Meteorology, the physics of the
atmosphere.
4. The Biosphere. The Biosphere is the study of the life forms on the
earth. It constitutes the science of Biology.
There is a continuous interaction with these different spheres.
The Vertical Divisions of the Atmosphere
The atmosphere is divided into four divisions based upon the
distribution of temperature in the vertical. They are:
1. The Troposphere. This is the lowest portion of the atmosphere.
Three-quarters of the total atmospheric mass is contained in the
troposphere. It is the area of clouds, storms, and convective motion. It
is sometimes called the weather sphere.
2) The Stratosphere. The region of the atmosphere called the
stratosphere, begins where the temperature stops decreasing with
altitude and becomes constant. It is in the stratosphere that ozone is
located.
3) The Mesosphere. The mesosphere begins at about 47 km where
the temperature no longer increases with height because there is no
more ozone above this level to absorb any ultraviolet radiation from
the sun.
4) The Thermosphere. The Thermosphere is the top layer of the
atmosphere. But because there are so few molecules at this height,
very little energy will be transferred to anything.
To go to another chapter, return to the table of contents by
clicking on this sentence.
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