Chapter 6 Air Pressure and Winds
6.1 Pressure
The difference in pressure between two points on the surface of the earth is
responsible for the winds that blow from the higher pressure area to the lower
pressure area. These winds in turn transfer heat and moisture from one place on
the earth to another. What is pressure and how does it relate to the winds?
Pressure is defined as the force acting per unit surface area. We write this
mathematically as
p= F
A
(6.1)
The unit for pressure is a newton/meter2 in the International System of Units while
in the British engineering system the units are lb/in.2, which is sometimes denoted
by psi.
Pressure exerted by a fluid is easily determined with the aid of figure 6.1,
which represents a pool of water. We want to determine the pressure p at the
bottom of the pool caused by the water in the pool. By our definition, equation 6.1,
water
p
Figure 6.1 Pressure in a pool of water.
the pressure at the bottom of the pool is the force acting on a unit area of the
bottom of the pool. But the force acting on a unit area at the bottom of the pool is
caused by the weight of all the water above it, shown as the hatched area in the
figure. Thus,
p = F = weight of water
(6.2)
A
area
p = w = mg
A
A
(6.3)
We have set the weight w of the water equal to mg in equation 6.3, where m is the
mass of the water and g is the acceleration due to gravity. The mass of the water in
the pool, is given by
Chapter 6 Air Pressure and Winds
m = ρV
(6.4)
where ρ is the density of the water in the pool, and V is the volume of the water in
the pool. The volume of all the water in the pool is just equal to the area A of the
bottom of the pool times the depth h of the water in the pool, that is,
V = Ah
(6.5)
Substituting equations 6.4 and 6.5 into equation 6.3 gives for the pressure at the
bottom of the pool:
p = mg = ρVg = ρAhg
A
A
A
Thus,
p = ρgh
(6.6)
(Although we derived equation 6.6 to determine the water pressure at the bottom of
a pool of water, it is completely general and gives the water pressure at any depth h
in the pool.) Equation 6.6 says that the water pressure at any depth h in any pool is
given by the product of the density ρ of the water in the pool, the acceleration due to
gravity g, and the depth h in the pool. Equation 6.6 is sometimes called the
hydrostatic equation.
Example 6.1
Pressure in a swimming pool. Find the water pressure at a depth of (a) 1.00 m, (b)
2.00 m, and (c) 3.00 m in a swimming pool.
Solution
The density of water is 1000 kg/m3.
(a) At a depth of 1.00 m the water pressure, found from equation 6.6, is
p = ρgh
= (1000 kg/m3)(9.80 m/s2)(1.00 m)
= 9,800 N/m2 = 1.42 lb/in2
(b) At a depth of 2.00 m the water pressure is
p = ρgh
= (1000 kg/m3)(9.80 m/s2)(2.00 m)
= 19,600 N/m2 = 2.84 lb/in2
(c) At a depth of 3.00 m the water pressure is
p = ρgh
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Chapter 6 Air Pressure and Winds
= (1000 kg/m3)(9.80 m/s2)(3.00 m)
= 29,400 N/m2 = 4.26 lb/in2
To go to this Interactive Example click on this sentence.
Notice from this example that the pressure exerted by the water becomes
greater the deeper you go into the water. The pressure at the greatest depth, 3 m, is
the greatest in the pool and is the same everywhere at this 3 m level. Hence, the
force exerted by the fluid is the same in all directions. That is, the force is the same
in up-down, right-left, or in-out directions. You experience this pressure when
swimming at a depth of 3.00 m as a pressure on your ears. As you swim up to the
surface, the pressure on your ears decreases because h is decreasing. Or to look at it
another way, the closer you swim up toward the surface, the smaller is the amount
of water that is above you. Because the pressure is caused by the weight of that
water above you, the smaller the amount of water, the smaller will be the pressure.
Just as there is a water pressure at the bottom of a swimming pool caused by
the weight of all the water above the bottom, there is also an air pressure exerted
on every object at the surface of the earth caused by the weight of all the air that is
above us in the atmosphere, figure 6.2. That is, we live at the bottom of an ocean of
air
p
Figure 6.2 Pressure at the bottom of an ocean of air.
air and that air exerts an atmospheric pressure on us, given by equation 6.1 as
p = F = weight of air
A
area
(6.7)
However we can not use the same result obtained for the pressure in the pool
of water, the hydrostatic equation 6.6, because air is compressible and hence its
density ρ is not constant with height throughout the vertical portion of the
atmosphere. The pressure of air at any height in the atmosphere can be found by
some involved calculation using the calculus. However, since such calculations are
beyond the scope of this course, we will use experimentation to determine the
pressure of the atmosphere.
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Chapter 6 Air Pressure and Winds
6.2 The Measurement of Atmospheric Pressure - The
Barometer
The pressure of the air in the atmosphere was first measured by Evangelista
Torricelli (1608-1647), a student of Galileo, by the use of a mercury barometer. A
long narrow tube is filled to the top with mercury, chemical symbol Hg. It is then
placed upside down into a reservoir filled with mercury, as shown in figure 6.3. The
h
po
po
Hg
p
Hg
Figure 6.3 A mercury barometer.
mercury in the tube starts to flow out into the reservoir, but it comes to a stop when
the top of the mercury column is at a height h above the top of the mercury
reservoir, as also shown in figure 6.3. The mercury does not empty completely
because the normal pressure of the atmosphere p0 pushes downward on the
mercury reservoir. Because the force caused by the pressure of a fluid is the same
in all directions, there is also a force acting upward inside the tube at the height of
the mercury reservoir, and hence there is also a pressure p0 acting upward as
shown in figure 6.3. This force upward is capable of holding the weight of the
mercury in the tube up to a height h. Thus, the pressure exerted by the mercury in
the tube is exactly balanced by the normal atmospheric pressure on the reservoir,
that is,
p0 = pHg
(6.8)
But the pressure of the mercury in the tube pHg, given by equation 6.6, is
pHg = ρHggh
(6.9)
Substituting equation 6.9 back into equation 6.8, gives
p0 = ρHggh
6-4
(6.10)
Chapter 6 Air Pressure and Winds
Equation 6.10 says that normal atmospheric pressure can be determined by
measuring the height h of the column of mercury in the tube. It is found
experimentally, that on the average, normal atmospheric pressure can support a
column of mercury 76.0 cm high, or 760 mm high. Using the value of the density of
mercury of 13,600 kg/m3, normal atmospheric pressure, determined from equation
6.10, is
kg
p 0 =  Hg gh = 13, 600 3 9.80 m2 (0.760 m )
m
s
= 1.013  105 N/m2 = 14.7 lb/in.2
Thus, the average or normal atmospheric pressure acting on us at the surface of the
earth is 1.013  105 N/m2, which is a rather large number as we will see presently.
In the study of meteorology, a different unit of pressure is usually employed,
namely the millibar, abbreviated mb. In terms of the unit of millibars, normal
atmospheric pressure is 1013 mb. Normal atmospheric pressure can also be
recorded as a height of 29.92 in. of Hg. On all surface weather maps in a weather
station, pressures are always expressed in terms of millibars.
To determine the actual pressure of the atmosphere from the reading of the
barometer certain corrections need to be made. A correction for the temperature of
the air must be made because if the temperature is high, the mercury in the
thermometer will expand and the barometer will read a higher pressure value than
the actual pressure of the air. Similarly if the temperature of the air is low, the
mercury in the barometer will contract and will move lower in the tube thereby
giving a lower pressure reading than that actually caused by the atmospheric air
pressure. The corrected reading is called the station pressure.
But pressure on mountains will always be lower than the pressure recorded
in valleys because the mountain is higher and hence there is less air above the
mountain than there is above the valley. Therefore in order to make possible direct
comparison of pressures at different altitudes, station pressure is converted to sealevel pressure. This is accomplished, see figure 6.4, by assuming that the
Station
height
above
sea level
sea level
Figure 6.4 Correction of station pressure to sea level pressure.
atmospheric column over the station extends all the way down to sea level and a
correction to represent the weight of the additional mass of air is added to the
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Chapter 6 Air Pressure and Winds
station pressure. In this way the air pressure recorded in every weather station in
the world is referred to the same mean sea level pressure.
The mercury barometer, with the appropriate corrections made, is thus a
very accurate means of determining air pressure. The value of 76.0 cm of mercury
or 1013 mb are only normal or average values. When the barometer is kept at the
same location and the height of the mercury column is recorded daily, the value of h
is found to vary slightly. When the value of h becomes greater than 76.0 cm of Hg,
the pressure of the atmosphere has increased to a higher pressure. It is then said
that a high-pressure area has moved into your region. When the value of h becomes
less than 76.0 cm of Hg, the pressure of the atmosphere has decreased to a lower
pressure and a low-pressure area has moved in. The barometer is extremely
important in weather observation and prediction because, as a general rule
of thumb, high atmospheric pressures usually are associated with clear
skies and good weather. Low-pressure areas, on the other hand, are usually
associated with cloudy skies, precipitation, and in general bad weather.
The mercury barometer, after certain corrections for instrument height above
sea level and ambient temperature, is an extremely accurate device to measure
atmospheric pressure and can be found in every weather station throughout the
world. Its chief limitation is its size. It must always remain vertical, and the glass
tube and reservoir are somewhat fragile. Hence, another type of barometer is also
used to measure atmospheric pressure. It is called an aneroid barometer, and the
face of one is shown in figure 6.4. It is based on the principle of a partially
29
Change
30
r
Rai
n
Fai
31
or
St
y
D
ry
my
Ve
r
28
27
26
Figure 6.4 An aneroid barometer.
evacuated, waferlike, metal cylinder called a Sylphon cell. When the atmospheric
pressure increases, the cell decreases in size. A combination of linkages and springs
are connected to the cell and to a pointer needle that moves over a calibrated scale
that indicates the pressure. The aneroid barometer is a more portable device that is
rugged and easily used, although it is originally calibrated with a mercury
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Chapter 6 Air Pressure and Winds
barometer. The word aneroid means not containing fluid. The aneroid barometer is
calibrated in both centimeters of Hg and inches of Hg. Hence, as seen in figure 6.4,
the pressure can be measured in terms of inches of mercury. Also note that regions
of high pressure (30 in. of Hg) are labeled to indicate fair weather, while regions of
low pressure (29 in. of Hg) are labeled to indicate rain or poor weather. In the
example of figure 6.4 the pointer needle is indicating a pressure a little above 30 in.
of Hg and indicating that the weather will be fair. Note that there is a second,
shorter needle pointing toward a pressure of approximately 29.2 in. of Hg. This
second needle gives the pressure tendency. When the barometer is read, this second
needle is rotated by a knob so as to be aligned with the pointer needle. When the
barometer is next read, let us say two or three hours later, the pointer needle may
have moved from its present position. As an example, in figure 6.4, let us say that 3
hours earlier the pointer needle and the shorter pressure tendency needle were
aligned where the shorter needle is presently located. This would mean that in the
last 3 hours the pressure has risen from 29.2 in. of Hg to 30.1 in. of Hg. Hence the
pressure tendency is increasing. This means that a higher pressure is moving into
your area. If the present position of the pointer needle was at say 28.8 in. of Hg, this
would indicate that the pressure is decreasing and a lower pressure area is moving
into your area. Since higher pressure area are usually associated with good weather
and lower pressure areas with bad weather, the pressure tendency is a good
indication of the type of weather that you are going to have.
As we go up into the atmosphere the pressure decreases, because there is less
air above us. The aneroid barometer will read smaller and smaller pressures with
altitude. Instead of calibrating the aneroid barometer in terms of centimeters of
mercury or inches of mercury, we can also calibrate it in terms of feet or meters
above the surface of the earth where this air pressure is found. An aneroid
barometer so calibrated is called an altimeter, a device to measure the altitude or
height of an airplane. The height of the plane is not really measured, the pressure
is. But in the standard atmosphere, a particular pressure is found at a particular
height above the ground. Hence, when the aneroid barometer measures this
pressure, it corresponds to a fixed altitude above the ground. The pilot can read this
height directly from the newly calibrated aneroid barometer, the altimeter.
Another type of instrument for measuring pressure is the barograph. The
barograph uses an aneroid barometer to measure atmospheric pressure. The
pressure is recorded on a chart placed on a drum that rotates. The chart gives a
complete record of atmospheric pressure variations for a day.
6.3 Some Examples of Atmospheric Pressure
Let us now look at some examples associated with atmospheric pressure.
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Chapter 6 Air Pressure and Winds
Example 6.2
Why you get tired by the end of the day. The top of a student’s head is approximately
circular with a radius of 3.50 inches. What force is exerted on the top of the
student’s head by normal atmospheric pressure?
Solution
The area of the top of the student’s head is found from
A = πr2 = π(3.50 in.)2 = 38.5 in.2
We find the magnitude of the force exerted on the top of the student’s head by
rearranging equation 6.1 into the form
F = pA
(6.11)
Hence,
F = 14.7 lb2 (38.5 in. 2 )
in.
= 566 lb
This is a rather large force to have exerted on our heads all day long.
To go to this Interactive Example click on this sentence.
Example 6.3
Atmospheric pressure on the walls of your house. Find the force on the outside wall
of a ranch house, 10.0 ft high and 35.0 ft long, caused by normal atmospheric
pressure.
Solution
The area of the wall of the house is given by
A = (length)(height)
2
= (35.0 ft)(10.0 ft) 144 in.
2
1ft
= 50,400 in.2
The force on the wall, given by equation 6.11, is
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Chapter 6 Air Pressure and Winds
F = pA = 14.7 lb2 (50, 400 in. 2 )
in.
= 740,000 lb
To go to this Interactive Example click on this sentence.
The force on the outside wall of the house in example 6.3 is thus 740,000 lb.
This is truly an enormous force. Why doesn’t the wall collapse under this great
force? The wall does not collapse because that same atmospheric air is also inside
the house. Remember that air is a fluid and flows. Hence, in addition to being
outside the house, the air also flows to the inside of the house. Because the force
exerted by the pressure in the fluid is the same in all directions, the air inside the
house exerts the same force of 740,000 lb against the inside wall of the house, as
shown in figure 6.5(a). The net force on the wall is therefore
Net force = (force)in − (force)out
= 740,000 lb − 740,000 lb
=0
Figure 6.5 Pressure on the walls in a house.
A very interesting case occurs when this net force is not zero. Suppose a
tornado, an extremely violent storm, were to move over your house, as shown in
figure 6.5(b). The pressure inside the tornado is very low. No one knows for sure
how low, because it is slightly difficult to run into a tornado with a barometer to
measure it. The pressure can be estimated, however, from the very high winds
associated with the tornado. A good estimate is that the pressure inside the tornado
is at least 10% below the actual atmospheric pressure. Let us assume that the
actual pressure is the normal atmospheric pressure of 1013 mb, then 10% of that is
101 mb. Thus, the pressure in the tornado is approximately
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Chapter 6 Air Pressure and Winds
2
1013 mb −101 mb= (912 mb) 14.7 lb/in. = 13.2 lb/in. 2
1013 mb
When the tornado goes over the house, the force on the outside wall is given by
F = pA = 13.2 lb2 (50, 400 in. 2 )
in.
= 665,000 lb
But the original air inside the house is still there and is still exerting a force of
740,000 lb outward on the walls. The net force on the house is now
Net force = 740,000 lb − 665,000 lb
= 75,000 lb
There is now a net force acting outward on the wall of 75,000 lb, enough to literally
explode the walls of the house outward. This pressure differential, with its
accompanying winds, accounts for the enormous destruction associated with a
tornado. Thus, the force exerted by atmospheric pressure can be extremely
significant.
It had once been customary to open the doors and windows in a house
whenever a tornado was in the vicinity, in the hope that a great deal of the air
inside the house would flow out through these open windows and doors. Hence, the
pressure differential between the inside and the outside walls of the house would be
minimized. Unfortunately many victims of tornadoes stopped following this
procedure, because tornadoes are spawned out of severe thunderstorms, which are
usually accompanied by torrential rain. Usually the first thing one does in a house
is to close the windows once the rain starts in order to avoid rain damage to the
house. Many times people suffered rather bad rain damage to the house through
the open windows and a tornado never even came that near to the house. I guess to
avoid law suits, no one recommends leaving the doors and windows open anymore.
6.4 Horizontal Distribution of Atmospheric Pressure
When the pressure is recorded at each weather station in the country, then
converted to a sea level pressure and plotted on a map, the comparison of the
pressure values at all these different stations will indicate small horizontal
differences in the pressure. As an example of the horizontal pressure distribution,
let us consider a weather 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. These lines are called isobars and can be seen in figure
6.6. An isobar is a line along which the pressure is constant. These different
pressures will be highly significant in forecasting the weather that will occur at
each of these different weather stations. On a world wide weather map these mean
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Chapter 6 Air Pressure and Winds
sea-level pressures will vary from pressures that are lower than 990 mb to
pressures that are higher than 1030 mb. Although low pressure and high pressure
are relative terms, regions on the surface of the earth that have pressures lower
than 1013 mb are called low pressure areas while regions that have pressures
higher than 1013 mb are referred to as high pressure areas. Horizontal pressure
Figure 6.6 A weather map.
differences result primarily from (1) the temperature differences that produce
different density differences and (2) dynamic causes that result from atmospheric
circulation. Also large amounts of water vapor in the air can cause slight variations
in air pressure because water vapor is less dense than the other constituents of the
air.
An isobar on a weather map 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 6.7(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 the mountain.
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Chapter 6 Air Pressure and Winds
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 6.7(b). (On a real
mountain and valley the contours would probably not be true circles.) If we were to
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 6.7 Contour lines on a topographical map.
place a large round bolder on the top of the mountain and give it a shove, it would
roll down the mountain into the valley below. If the mountain is very steep, the
velocity of the bolder will be large. If the mountain is shallow and not steep, the
velocity of the bolder down the mountain will be much smaller.
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
6.8(a). The high-pressure region can be visualized as a mountain and the lowpressure region as a valley in figure 6.8(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. Just
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Chapter 6 Air Pressure and Winds
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 6.8 High and low atmospheric pressure.
as the velocity of the ball rolling down the real mountain depends upon the
steepness of the mountain, the velocity of the wind as it rolls down the pressure
mountain depends on the steepness of the pressure mountain. The steepness of the
pressure mountain is determined by the pressure gradient which is defined as the
rate of change of atmospheric pressure between two points at the same elevation.
This is written mathematically as
p
p =
(6.12)
x
The symbol ∆ is called delta and it is a short hand mathematical notation that
means “change in”. Hence, the symbol ∆p means the “change in pressure p” while
the symbol ∆x means the “change in distance x”. The upside down delta symbol,
called del, when used with the pressure p and designated as p is the symbol used
to denote the pressure gradient. The pressure gradient p is proportional to the
difference in pressure and is the immediate cause of horizontal air movement, that
is, the wind.
The pressure gradient is the isobaric slope of the pressure mountain. When
the isobars are packed very closely together, a large pressure gradient p results.
This large pressure gradient, which corresponds to a steep slope, causes very large
winds, that is, winds of very high velocity. When the isobars are spaced very far
apart, a small pressure gradient p results. This small pressure gradient
corresponds to a shallow slope, and causes very light winds, that is, winds of very
low velocity.
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Chapter 6 Air Pressure and Winds
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 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) and is the pressure
gradient per unit mass, it is directed from the high-pressure area to the lowpressure area. It is effectively the slope of the pressure mountain-valley.
6.5 The Coriolis Force
If the earth were not rotating, the air would flow perpendicular to the
isobars, directly from the high pressure area to the low pressure area. However, the
earth does rotate, and the rotation of the earth causes air to be deflected to the right
of its original path in the northern hemisphere. The deflection of air to the right of
its path in the northern hemisphere is called the Coriolis effect. The Coriolis effect
arises because the rotating earth is not an inertial coordinate system. For smallscale motion the rotating earth approximates an inertial coordinate system.
However, for large-scale motion, such as the winds, the effect of the rotating earth
must be taken into account.
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
6.9(a).
N
N
Equator
Equator
S
S
(a) Nonrotating earth.
(b) Rotating Earth
Figure 6.9 The Coriolis force.
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Chapter 6 Air Pressure and Winds
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 projectile veer away to the right of its initial path, figure 6.9(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 given by
CF = 2vΩ sin φ
(6.13)
where CF is the Coriolis force per unit mass of air, v is the speed of the wind at the
particular location, Ω is the angular velocity of the earth, and φ is the latitude on
the earth where the observation is being made. Thus, the Coriolis force depends on
the speed of the air (the greater the speed of the air the greater the force that will
pull the air to the right, the lower the speed, the smaller 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. That is the Coriolis force is zero at the equator. As the latitude angle
φ increases, the Coriolis force increases until for φ = 900, sinφ = 1, and hence the
maximum force and deflection occur at the pole.
We should point out that just as air is deflected to the right of its path in the
northern hemisphere by the Coriolis force, air will be deflected to the left of its path
in the southern hemisphere, figure 6.10. Imagine a missile fired from the south pole
toward some location A on the equator. It starts out toward the position A at the
equator, but while in flight the earth is rotating. By the time the missile reaches
the equator, the position A has rotated eastward away from its original position. To
the observer at A he sees the missile as though it was deflected to the left of its
original path. The magnitude of the Coriolis force in the southern hemisphere is the
same as in the northern hemisphere, only the direction is different. Figure 6.10
shows that air is deflected by the Coriolis force to the right of its original path in
the northern hemisphere and to the left of its original path in the southern
hemisphere. The amount of the deflection is governed by the speed v of the air and
the latitude angle φ as given in equation 6.13.
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Chapter 6 Air Pressure and Winds
N
Equator
S
Figure 6.10 The Coriolis force in both hemispheres.
6.6 Wind Flow in a Low Pressure Area
Taking the Coriolis force into account, let us now describe the motion of the
air as it moves toward a low-pressure area. The pressure gradient force PGF
initiates the movement of the parcel of air, located at the point A, figure 6.11(a),
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 6.11 Winds in a low-pressure area.
along a path that is 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 the air parcel now
A more detailed analysis by Newton’s second law would give
1
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Chapter 6 Air Pressure and Winds
moves parallel to the isobars. Notice that the air moves counterclockwise in a lowpressure area. A low pressure area is sometimes called a cyclone and this
counterclockwise flow in a low pressure area in the northern hemisphere is called
cyclonic flow.
A simple practical approach to locating the center of a low pressure area is
given by Buys-Ballot rule. Buys-Ballot rule says that if you stand with your back to
the wind in the northern hemisphere, the pressure on your left is less than the
pressure on your right. This can be seen in figure 6.12.
L
Left
Right
Figure 6.12 Buys-Ballot rule.
As the air moves over the ground, there is a frictional force f that acts on the
air which is directed opposite to the direction of motion of the air. This frictional
force is responsible for the slowing down of the air which eventually causes a
change in direction of the wind. This is shown in figure 6.11(b).
a = F = PGF + CF
m
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
or
0 = PGF + CF
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. The balanced flow of air in a low pressure area taking the centripetal
acceleration into account is called the gradient wind.
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Chapter 6 Air Pressure and Winds
This effect can be explained by noting that the Coriolis force CF is a function
of the wind speed v, as seen in equation 6.13. 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 PGF and the Coriolis force CF. Since the
pressure gradient force is now larger than the Coriolis force, the air parcel moves
across the isobars at an angle toward the low-pressure area. The combined result of
the pressure gradient force PGF, the Coriolis force CF, and the frictional force f,
causes the air to spiral into the low-pressure area, as seen in figure 6.11(b). Over
land the wind crosses the isobars an angle of about 450 while over large water areas
the wind crosses the isobars an angle of about 100 to 200.
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, as shown in figure 6.13.
L
Figure 6.13 Relation between convergence of air at the surface and
ascending air aloft.
That is, there is a general convergence of air into the low pressure area at the
surface and then vertical motion upward into the atmosphere. At some greater
height in the atmosphere the rising air will diverge away to keep the low pressure
area at the surface from filling up. Hence, there must be a region of divergence aloft
to maintain the convergence at the surface.
Now recall that 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. But for a gas to expand the gas must do work.
Since there is no heat added to, or taken away from this rising air, the air is
expanding adiabatically. But the work done in the expansion causes a decrease in
the internal energy of the gas. Hence, the rising air cools as it expands because the
energy necessary for the gas to expand comes from the internal energy of the gas
itself. Hence the temperature of the air decreases as the air expands and the rising
air cools.
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Chapter 6 Air Pressure and Winds
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 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.
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 larger, As they collide and coalesce with still other cloud droplets
they 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.
6.7 Wind Flow in a High Pressure Area
Everything we said about the low-pressure area is reversed for a highpressure area. The pressure gradient force points away from the high-pressure
area. As the air starts out of the high-pressure area at the point A, figure 6.14(a), it
is moving along a path that is perpendicular to the isobars. 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 size. The pressure gradient force PGF is now
greater than the Coriolis force CF, and the air starts to spiral out of the
H
A
H
P.G.F
C.F C.F
v
C.F
B
v
f
P.G.F
P.G.F
(a) No friction
(b) Friction
Figure 6.14 Winds in a high-pressure area.
12The 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.
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Chapter 6 Air Pressure and Winds
high-pressure area, figure 6.14(b). A high pressure area is sometimes called an
anticyclone and the clockwise flow in the high pressure area in the northern
hemisphere is often referred to anticyclonic flow.
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. The only place that it can
come from is from the air aloft. That is, air aloft moves downward into the highpressure area at the surface. Thus, there is vertical motion downward in a highpressure area. This subsiding air, and the surface divergence of that air, is shown
in figure 6.15.
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.
H
Figure 6.15 Relation between divergence of air at the surface and subsiding
air aloft.
Now when you look at your TV weather map, look for the low- and highpressure 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.
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Chapter 6 Air Pressure and Winds
6.8 Wind Observations
For a complete description of the wind we need to measure both the wind
direction and the wind speed. To measure the direction of the wind a wind vane is
used. A wind vane usually consists of a metallic arrow that is mounted on a pole
and is free to rotate, as shown in figure 6.16. The pole is usually attached to the roof
of a building. Since the tail of the arrow is bigger than the tip of the arrow, when
the wind hits the arrow it pushes against the tail of the arrow causing it to rotate.
The arrow will rotate until the tip of the arrow points into the wind. An electrical
N
W
E
S
Figure 6.16 A weather vane.
connection is usually made on the wind vane which shows the direction of the wind
inside the weather station. Since the arrow points into the direction from which the
wind flows, the winds are named for the direction from which they come. That is, a
wind that blows from the south to the north is called a south wind, whereas a wind
the blows from the north to the south is called a north wind. To account for all the
different possible directions for the wind flow, a compass rose is used as shown in
figure 6.17. If a wind blows from the east it is called an easterly
NW
N
360000
NNE
NE
ENE
900 E
W 2700
SW
1800
S
SE
Figure 6.17 The compass rose and wind directions.
wind. If the wind blows half way between the north and the east it is called a
northeast wind. If the wind blows half way between the south and the east it is
called a southeast wind. If the wind blows half way between the north and the west
it is called a northwest wind, etc. If the wind blows halfway between north and
northeast it is called a north-northeast wind. If the wind blows halfway between
northeast and east it is called a east-northeast wind. Using this type of description
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Chapter 6 Air Pressure and Winds
we can describe the direction of the wind for all different directions. For even more
detailed description of the wind direction we can use the angles of the compass rose
for the wind direction. Thus an east wind would flow from a direction of 900, a south
wind from 1800, a west wind from 2700, and a wind from the north as either 00 or
3600. Some arbitrary wind might blow from an angle of 350. So to determine the
direction of the wind, all you have to do is to look at the weather vane and see in
what direction it is pointing. That direction is the direction from which the wind is
blowing.
To determine the speed of the wind we use a device called an anemometer.
An example of an anemometer is shown in figure 6.18. It consists of a long pole
usually attached to the roof of a building. Connected to it are three metal rods that
extend outward in the horizontal direction. At the end of each rod is a little cup. As
the wind blows past the anemometer it gets caught in the little cup and causes the
cup to rotate around the pole. The greater the speed of the wind the greater the
rotation rate of the anemometer cups. The rotation is transferred to a coil below
Figure 6.18 An anemometer for measuring wind speed.
that rotates in a magnetic field. The rotating coil in the magnetic field causes a
current to flow in a meter that is calibrated to give the wind speed. In this way the
wind speed is determined. The standard unit in meteorology for expressing the wind
speed is the knot. A knot is defined as a nautical mile per hour, which is equivalent
to 1.15 statute miles per hour. (Recall that a statute mile is equal to 5280 ft. while a
nautical mile is equal to 6080 ft, and also that 1 mph = 0.87 knots.)
The symbols used on a weather map to indicate the wind speed are shown in
figure 6.19. The horizontal straight line in that figure represents the direction of
the wind velocity and the marker on the end of the line indicates the wind speed.
One line indicates the wind speed is 10 knots, two lines 20 knots, three lines 30
knots, etc. A shorter line indicates a wind speed of 5 knots and when combined with
say two longer lines gives 25 knots. In this way any wind speed up to 50 knots can
be designated on a weather map. A fifty knot wind has a small blackened triangle at
the end of the line. Two triangles represents 100 knots and three triangles 150
knots. A triangle and two lines would represent a speed of 70 knots. Thus different
combinations of triangles and lines can show any wind speed above 50. Two
examples of a weather station marked on a map showing the wind speed and
direction are shown on the right of figure 6.19.
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Chapter 6 Air Pressure and Winds
5 Kts
10 kts
15 kts
20 kts
25 kts
Station
50 Kts
60 Kts
65 Kts
70 Kts
30 kts
75 Kts
35 kts
100 Kts
40 kts
150 Kts
0
25 Kt wind from 270
or a westerly wind
of 25 Kt
Station
0
20 Kt wind from 315
or a northwest wind
of 20 Kt
Figure 6.19 Wind speed
Some maps are drawn to show the distribution of wind speeds throughout the
country. On such a map lines are drawn connecting points that have the same wind
speed. Lines that connect points that have the same wind speed are called isotachs.
6.9 Winds Aloft
Wind direction changes with altitude. As we get away from the surface of the
earth the wind does not slide over the earth’s surface and therefore friction does not
slow down the winds. Hence the winds become more nearly parallel to the isobars
and wind speed becomes more nearly geostrophic.
A graph of the 500 mb surface is shown in figure 6.20. At every point on this
map the pressure is 500 mb. This map is called a constant pressure map. The 500
mb surface rises and falls throughout the atmosphere and the height of the 500 mb
surface at any point is measured by a height in meters. So instead of actually
drawing isobars on a constant height map such, as is done at the surface, contours
of different heights are drawn on a constant pressure surface. The effect is still the
same. Where the contour number is high, this represents high pressure, where the
contour number is low, this will represent low pressure. Notice that the map does
not have all the closed low and high pressure centers that are observed on the
surface weather map. Instead we have ridges and troughs in the contour field. A
ridge corresponds to a high pressure and a trough to low pressure. Notice that the
winds aloft blow more nearly parallel to the contour lines than they do on the
surface map.
Upper air winds are determined by a weather balloon which is tracked either
optically or by radar. To measure the wind optically with a balloon, called a pilot
balloon or sometimes just referred to as a pibal, the balloon is released from the
surface and then tracked by a theodolite. A more general way to track the balloon is
by radar, and when this is done it is called a Rawin observation.
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Chapter 6 Air Pressure and Winds
Figure 6.20 A 500 mb map.
Strong convection currents cause turbulence in the air aloft. Along
boundaries between air streams having different speeds wind shear develops
creating violent turbulence.
Along the boundary where cold air advances against warmer air, the lighter
warmer air will rise and spread out above the heavier colder air below, so that at
times the winds aloft may be blowing directly opposite the winds below at the
surface.
It is observed that there is a diurnal variation of wind speed at the surface of
the earth. That is, in the early afternoon the surface wind speeds are at their
maximum, while in the early morning hours just before sunrise the winds will be at
their lowest value. This diurnal variation of wind speed comes about because of the
convection of the air by the heating of the surface during the day light hours. The
convection causes an exchange of the slower air below with the faster air aloft. At
night the slower air at the surface stays where it is because there is now no thermal
convection.
6.10 Local Winds
There are many winds that occur because of the local topography of the area
and these are usually referred to as local winds. Some of these local winds and their
description are shown below.
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Chapter 6 Air Pressure and Winds
The Sea Breeze
Sea breezes occur on clear days in summer, along seacoasts or large inland water
bodies. Because the specific heat of water is so much greater than the specific heat
of land, the water is able to absorb the same quantity of heat from the sun with a
smaller temperature change than the temperature change of the land. Hence, the
land heats more rapidly than the water, and the temperature of the air over the
land becomes greater than the temperature of the air over the water. We can
describe the effect of this in two ways. From the point of view of the temperature of
the air, the hot air over the land rises into the atmosphere. The colder air over the
ocean now blows on shore to replace this rising air over the land. The air aloft over
the ocean now descends to the ocean surface to replace the air that has just blown
on shore. The hot air over the land which rose up into the atmosphere, now moves
out over the ocean to replace the air that just descended to the ocean surface. Hence
a convection cycle has occurred and is shown in figure 6.21. The net result of the
sea breeze is to bring cool refreshing winds from over the ocean onshore to the hot
summer land. This is why so many people flock to the beaches in the summertime.
This is why Long Island is such a popular place to live because it is relatively cool
in the usual very hot days of summer.
H
L
L
Sea Breeze
Hot land
H
Cool sea
Figure 6.21 The sea breeze.
The sea breeze can also be explained on the basis of a pressure differential
between the air over the ocean and the air over the land. The hotter air over the
land expands and becomes less dense than it was previously. This expanding air is
less dense and hence the pressure over the land decreases. The colder air over the
ocean is more dense and hence its pressure is greater than the pressure of the air
over the land. Thus a local low pressure area has developed over the land and a
local high pressure area has developed over the ocean. The winds now flow from the
high pressure over the ocean to the low pressure over the land thus bringing cool
air from the ocean onto the hot land, giving us the sea breeze.
The sea breeze is considered a local wind because the maximum distance
inland that the wind blows is only about 50 km (30 miles), and the maximum depth
of the sea breeze is only about 1000 m. The sea breeze begins in the late morning
hours, from about 11 AM to 12 noon, and moves inland to decrease the afternoon
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Chapter 6 Air Pressure and Winds
temperatures. Thus, areas along the coast will have cooler temperatures than areas
inland.
At night the process is reversed, and we have what is called the land breeze,
see figure 6.22. At night the land cools more rapidly than the ocean, leaving warmer
air over the ocean than over the land. Now the warmer air over the ocean rises and
the cooler air over the land blows out to replace this rising air. Air aloft over the
land descends to replace the departing cool air over the land. Again a convection
cycle is developed and is seen in figure 6.22. The land breeze is not as strong as the
sea breeze and not as extensive. The maximum wind occurs in the early morning
hours and dies out soon after sunup.
L
H
Land Breeze
H
Cool land
L
Warm sea
Figure 6.22 The land breeze.
Mountain and Valley Breeze.
People who live near mountains or valleys experience a local wind called a
mountain wind or a valley wind. On clear cool nights the mountain radiates very
much energy away and the air in contact with the mountain becomes very cool. This
cool air flows down the mountain into the valley below and is called a mountain
breeze.
On warm sunny days the mountain receives a good deal of radiation from the
sun and warms rapidly. The warm air in contact with the mountain rises and the
air below in the valley flows up the side of the mountain to replace this air giving
rise to a valley breeze.
6.11 Global Pressure Distribution and Circulation
Convection is the main mechanism of thermal energy transfer in the
atmosphere. On a global basis, the nonuniform temperature distribution on the
surface of the earth causes convection cycles that result in the prevailing winds. If
the earth were not rotating a huge convection cell would be established as shown in
figure 6.23(a). The overall pressure distribution on the surface of the earth occurs
because of the uneven temperature distribution on the surface of the earth. The
equator is the hottest portion of the earth because it gets the maximum radiation
from the sun. The hot air at the equator expands and becomes less dense. Recall
that atmospheric pressure is caused by the weight of all the air over a unit area of
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Chapter 6 Air Pressure and Winds
the surface. If the air is less dense, there is less of it and hence its weight per unit
area is going to be less. Thus the pressure at the equator is going to be low
pressure. We call this low pressure area the equatorial low. Air both north and
south of the equator will flow into this equatorial low. This convergence of
surface air at the equator causes the air to rise into the atmosphere.
While this is happening at the equator let us look at the north pole. The air
at the north pole is very cold because of the small amount of solar radiation
reaching the surface. If the air is very cold, it contracts and becomes more dense.
cold cold
Polar High
hot
Equatorial Low
hot
Polar High
(a) Non-rotating Earth
(b) Rotating Earth
Figure 6.23 Global pressure distribution.
Recall that atmospheric pressure is caused by the weight of all the air over a unit
area of the surface. If the air is more dense, there is more of it and hence its weight
per unit area is going to be greater. Thus the pressure at the pole is going to be
high pressure. This region of high pressure is called the polar high. As we
have seen, air flows out of a high pressure area. Hence the cold surface air at the
pole now flows outward toward the equator to replace the hot rising surface air at
the equator. Air aloft over the poles descends to replace the air at the surface that
just moved toward the equator. The initial rising air at the equator flows toward
the pole, completing the convection cycle. The net result of the cycle is to bring hot
air at the surface of the equator, aloft, then north to the poles, returning cold air at
the polar surface back to the equator.
This simplified picture of convection on the surface of the earth is not quite
correct, because the effect produced by the rotating earth, the Coriolis Effect, has
been neglected. As you recall, the Coriolis effect is caused by the rotation of the
earth. When the Coriolis effect is applied to the global circulation of air in the
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Chapter 6 Air Pressure and Winds
atmosphere, it causes winds in the northern hemisphere to be deflected to the right
of their original path. The global convection cycle described above still occurs, but
instead of one huge convection cell, there are now three smaller ones in this same
area, figure 6.23(b). The reason for this is because as the air aloft at the equator
flows north it converges because of the spherical shape of the earth. As an example,
the distance between 10 degrees of longitude at the equator is greater than the
distance between the same 10 degrees of longitude at 300 north latitude. So all that
air at the equator converges together as it heads north. Also, because of the Coriolis
effect, that same air is deflected to the right of its path and eventually flows in an
easterly direction at approximately 300 north latitude. The piling up, or
convergence, of all this air at this latitude causes the air aloft to sink to the surface,
figure 6.23(b), where it forms a semipermanent high pressure area called the
subtropic high, figure 6.24(a). The air at the surface that flows north from this
high pressure area is deflected to the right of its path toward the east, thereby
producing the mid-latitude westerlies, figure 6.23(a) and figure6.24(b), or as it is
sometimes called the prevailing westerlies. The air at the surface that flows south
from this subtropic high pressure is also deflected to the right of its path toward the
southwest producing the northeast trade winds, also shown in figures 6.23(b) and
6.24(b). The winds from the polar high flowing south at the surface of the earth are
Polar High
Subpolar Low
Subtropic High
H
600
L
300
Equatorial Low
Westerlies
H
L
Subtropic High
Polar
Easterlies
North East
Trade Winds
Doldrums
Subpolar Low
Polar High
(a)
(b)
Figure 6.24 (a) Global pressure distribution (b) Global winds
deflected to the right of their path and become the polar easterlies as shown in
figures 6.23(b) and 6.24(b). The converging air from the polar easterlies and
the mid-latitude westerlies form a low pressure area, called the subpolar
low. Thus air flowing north from the subtropic high and the air flowing south from
the polar high converge and form the subpolar low. Thus because of the
rotation of the earth, there are now four permanent air pressures on the
surface of the earth, the polar high, the subpolar low, the subtropical high
and the equatorial low, as shown in figure 6.24(a). Thus, it is the nonuniform
temperature distribution on the surface of the earth that causes the
pressure distribution that is responsible for the global winds.
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Chapter 6 Air Pressure and Winds
The westerlies and the polar easterlies meet and converge at the subpolar
lows or polar fronts, figure 6.23(b). Here there is frequently a great contrast
between the temperature of the winds from subtropical and polar source regions,
giving rise to the cyclonic vortices or lows that are carried along in the westerlies.
Although the winds in these cyclonic storms may blow from any direction, the
prevailing westerlies causes the entire low pressure area, with all its clouds and
precipitation, to move from the west toward the east. The weather we get today is
the weather that the people west of us got yesterday, along with any modifications
caused by the strengthening or weakening of the storm system. We will see more
about this in the next chapter.
The Language of Meteorology
Pressure - The magnitude of the normal force acting per unit surface area. The
difference in pressure between two points on the surface of the earth is responsible
for the winds that blow from the higher pressure area to the lower pressure area.
The hydrostatic equation - An equation that gives the pressure of a fluid at a
particular depth.
Barometer - An instrument that measures atmospheric pressure.
Isobar - a line on a weather map, connecting those points whose pressures are the
same.
Coriolis effect - the rotation of the earth causes air to be deflected to the right of
its original path in the northern hemisphere.
Low pressure area - air spirals counterclockwise into a low-pressure area at the
surface of the earth. There is vertical motion upward in a low-pressure area. The
cooling of this adiabatically expanding air causes the formation of clouds,
precipitation, and general bad weather.
High pressure area - air flows clockwise around a high-pressure area, and spirals
out of the high pressure area at the surface of the earth. The air aloft descends to
replace the air leaving at the surface. 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.
Wind Direction - Winds are named for the direction from which they come.
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Chapter 6 Air Pressure and Winds
Anemometer - A device used to determine the speed of the wind.
Knot - The standard unit in meteorology for expressing the wind speed. A knot is
defined as a nautical mile per hour.
Global Pressure Distribution and Circulation - On a global basis, the
nonuniform temperature distribution on the surface of the earth causes convection
cycles that result in the prevailing winds. The four main pressure areas in the
atmosphere are the Polar High, the Subpolar Low, the Subtropic High, and the
Equatorial Low.
Polar Easterlies - The winds from the polar high flowing south at the surface of
the earth are deflected to the right of their path and become the polar easterlies
Mid-latitude Westerlies or Prevailing Westerlies - The air at the surface that
flows north from the subtropic high pressure area is deflected to the right of its
path toward the east, thereby producing these westerly winds .
Northeast Trade Winds - The air at the surface that flows south from this
subtropic high pressure is also deflected to the right of its path toward the
southwest producing these northeast trade winds.
Summary of Important Equations
p= F
A
(6.1)
Hydrostatic equation
p = ρgh
(6.6)
Force
F = pA
(6.11)
CF = 2vΩ sin φ
(6.13)
Pressure
Coriolis force
Questions for Chapter 6
1. When you fly in an airplane you find that your ears keep “popping” when
the plane is ascending or descending. Explain why.
2. Using a barometer and the direction of the wind, describe how you could
make a reasonable weather forecast.
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Chapter 6 Air Pressure and Winds
3. A pilot uses an aneroid barometer as an altimeter that is calibrated to a
standard atmosphere. What happens to the aircraft if the temperature of the
atmosphere does not coincide with the standard atmosphere?
To go to another chapter, return to the table of contents by
clicking on this sentence.
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