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Section 19.2
19.2 Pressure Centers
and Winds
1 FOCUS
Section Objectives
19.5
Key Concepts
Describe how winds blow
around pressure centers in
the Northern and
Southern Hemispheres.
What are the air pressure
patterns within cyclones
and anticyclones?
How does friction control
net flow of air around a
cyclone and an anticylone?
Vocabulary
◆
◆
◆
◆
◆
◆
◆
cyclone
anticyclone
trade winds
westerlies
polar easterlies
polar front
monsoon
How does the atmosphere
attempt to balance the
unequal heating of Earth’s
surface?
Reading Strategy
Comparing and Contrasting Copy the
table below. As you read about pressure
centers and winds, fill in the table indicating
to which hemisphere the concept applies. Use
N for Northern Hemisphere, S for Southern
Hemisphere, and B for both.
Cyclones rotate
counterclockwise.
a.
?
Net flow of air is inward
around a cyclone.
b.
?
Anticyclones rotate
counterclockwise.
c.
?
Coriolis effect deflects
winds to the right.
d.
?
19.6
19.7
19.8
Explain how winds blow
around pressure centers in
the Northern and Southern
Hemispheres.
Describe the air pressure
patterns within cyclones and
anticyclones.
Describe how friction controls
the net flow of air around a
cyclone and an anticyclone.
Explain how the unequal
heating of Earth’s surface
affects the atmosphere.
Reading Focus
Build Vocabulary
P
ressure centers are among the most common features on any
weather map. By knowing just a few basic facts about centers of high
and low pressure, you can increase your understanding of present and
forthcoming weather. You can make some weather generalizations
based on pressure centers. For example, centers of low pressure are frequently associated with cloudy conditions and precipitation. By
contrast, clear skies and fair weather may be expected when an area is
under the influence of high pressure, as shown in Figure 6.
Figure 6 These sunbathers at
Cape Henlopen, Delaware, are
enjoying weather associated with
a high-pressure center.
Concept Map Have students make a
concept map using the term global winds
as the starting point. All the vocabulary
terms in this section except cyclone and
anticyclone should be used. Have
students include the definitions of the
vocabulary terms in their concept maps.
Reading Strategy
a. N
c. S
Highs and Lows
Lows, or cyclones (kyklon ⫽ moving in a circle) are
centers of low pressure. Highs, or anticyclones, are
centers of high pressure.
In cyclones, the pressure decreases from the outer isobars toward the
center. In anticyclones, just the opposite is the
case—the values of the isobars increase from the
outside toward the center.
L2
L2
b. B
d. N
2 INSTRUCT
Highs and Lows
Build Science Skills
Air Pressure and Wind
537
L2
Use Models Have
students use a globe
to review and demonstrate the Coriolis
effect. One student can rotate the globe
while the other uses a pointer to show a
flow of air moving straight down from a
pole to the equator. Students can also
use the model to compare the direction
in which airflow is deflected in the
Northern and Southern Hemispheres.
Ask: Seen from Earth’s orbit, what is
the relationship between Earth’s
surface and the line along which the
air is flowing? (Earth rotates beneath the
line of airflow.) Seen from Earth’s
surface, what appears to be
happening? (Earth’s rotation makes
it appear that the airflow is deflected to
one side—to the right in the Northern
Hemisphere, to the left in the Southern
Hemisphere.)
Visual, Verbal
Air Pressure and Wind 537
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Section 19.2 (continued)
1012
1012
1008
1004
1004
1008
1012
1016
1008
1000
Warm Air Rises
L2
992
1008
Materials 2 meter sticks, 2 large paper
grocery bags, string, tape, lamp with
incandescent bulb
Procedure Tape string to the bottom
of each bag. Tie the two strings to a
meter stick, as far apart as possible. The
bags should be hanging upside down.
Use more string to hang this meter stick
at its balance point from another meter
stick that rests between two student
desks. Place the lamp beneath one of
the bags and switch it on.
996
(Anticyclone)
Purpose Students see that air rises
upward as it is warmed by a heat source.
(Cyclone)
1012
1016
1020
Figure 7 This map shows cyclonic
and anticyclonic winds in the
Northern Hemisphere.
Expected Outcomes The lamp heats
the air inside the bag, causing it to rise
above the other bag and showing that
warm air rises.
Visual, Logical
L2
Cyclonic and Anticyclonic Winds You learned that the two
most significant factors that affect wind are the pressure gradient and
the Coriolis effect. Winds move from higher pressure to lower pressure and are deflected to the right or left by Earth’s rotation.
When
the pressure gradient and the Coriolis effect are applied to pressure
centers in the Northern Hemisphere, winds blow counterclockwise
around a low. Around a high, they blow clockwise. Notice the wind
directions in Figure 7.
In the Southern Hemisphere, the Coriolis effect deflects the winds
to the left. Therefore, winds around a low move clockwise. Winds
around a high move counterclockwise.
In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow
of air outward around an anticyclone.
Weather and Air Pressure Rising air is associated with cloud
formation and precipitation, whereas sinking air produces clear skies.
Imagine a surface low-pressure system where the air is spiraling
inward. Here the net inward movement of air causes the area occupied
by the air mass to shrink—a process called horizontal convergence.
Whenever air converges (or comes together) horizontally, it must
increase in height to allow for the decreased area it now occupies. This
increase in height produces a taller and heavier air column. A surface
low can exist only as long as the column of air above it exerts less pressure than does the air in surrounding regions. This seems to be a
paradox—a low-pressure center causes a net accumulation of air,
which increases its pressure.
Students may have the misconception
that no outside factor except heat is
required to cause masses of warm air
to rotate as they rise upward in the
atmosphere. Point out that the cyclonic
motion in low-pressure cells is primarily
a result of the Coriolis effect. As a followup, ask: What is the primary cause of
the anticyclonic motion of highpressure cells? (Coriolis effect )
Verbal, Logical
With what type of weather is rising air associated?
538 Chapter 19
Customize for Inclusion Students
Visually Impaired Help visually impaired
students compare cyclones and anticyclones
by using their hands to model the motions of
these phenomena. Explain that, in the Northern
Hemisphere, cyclonic winds blow inward and
counterclockwise around a low-pressure
center. Have students use one hand to
538 Chapter 19
represent Earth’s surface. This hand remains
stationary. They can use the other hand to
make the counterclockwise, inward motion of
a cyclonic wind. Then have students use the
same hand to make the clockwise, outward
motion of an anticyclonic wind in the
Northern Hemisphere.
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Build Reading Literacy
Airflow Patterns, Surface and Aloft
Refer to p. 530D, which provides the
guidelines for making inferences.
Divergence aloft
Convergence aloft
Rising
air
Subsiding
air
Surface
convergence
CYCLONIC FLOW
L1
Surface
divergence
Figure 8 Air spreads out,
or diverges, above surface
cyclones, and comes together,
or converges, above surface
anticyclones.
Applying Concepts Why is fair
weather associated with a high?
ANTICYCLONIC FLOW
In order for a surface low to exist for very long, converging air at
the surface must be balanced by outflows aloft. For example, surface
convergence could be maintained if divergence, or the spreading out of
air, occurred above the low at a rate equal to the inflow below. Figure
8 shows the relationship between surface convergence (inflow) and
divergence (outflow) needed to maintain a low-pressure center. Surface
convergence around a cyclone causes a net upward movement. Because
rising air often results in cloud formation and precipitation, a lowpressure center is generally related to unstable conditions and stormy
weather.
Like cyclones, anticyclones also must be maintained from above.
Outflow near the surface is accompanied by convergence in the air
above and a general sinking of the air column, as shown in Figure 8.
Make Inferences After students have
finished reading this section, ask:
What does sunlight have to do with
the occurrence of rainy weather?
(Sunlight warms the atmosphere.
Differential warming creates pressure
gradients, which result in the formation
of high- and low-pressure centers.
Winds blow toward low-pressure centers,
bringing moist air in which clouds can
form.)
Verbal, Logical
L1
Use Visuals
Figure 8 After students have examined
the illustration, ask: Where does air
enter a cyclonic flow? (at Earth’s
surface) Where does air leave a
cyclonic flow? (aloft) Where does air
enter an anticyclonic flow? (aloft)
Where does air leave an anticyclonic
flow? (at Earth’s surface)
Visual, Verbal
Weather Forecasting Now you can see why weather reports
emphasize the locations and possible paths of cyclones and anticyclones. The villain in these reports is always the low-pressure center,
which can produce bad weather in any season. Lows move in roughly
a west-to-east direction across the United States, and they require a
few days, and sometimes more than a week, for the journey. Their
paths can be somewhat unpredictable, making accurate estimation of
their movement difficult. Because surface conditions are linked to the
conditions of the air above, it is important to understand total atmospheric circulation.
Air Pressure and Wind
539
Facts and Figures
Accurate forecasts require that meteorologists
not only predict the movement of low-pressure
centers, but also determine if the airflow aloft
will intensify an embryo storm or act to suppress
its development. Surface cyclones would quickly
eradicate themselves—not unlike the incoming
rush of air that occurs when a vacuum-packed
can is opened—without divergence in the air
above. As a result, meteorologists must base
their forecasts on data from upper and lower
atmospheric conditions. Because of the close
relationship between conditions at the surface
and aloft, understanding of total atmospheric
circulation, particularly in the mid-latitudes, is
very important.
Answer to . . .
Figure 8 Convergence aloft causes a
column of air to sink, creating a zone
of high pressure and fair weather.
Sinking air is compressed and warmed
as it nears Earth’s surface, which
discourages cloud formation.
cloud formation and
precipitation
Air Pressure and Wind 539
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Section 19.2 (continued)
Global Winds
The underlying cause of wind is the unequal heating of Earth’s surface. In tropical regions, more solar radiation is received than is
radiated back to space. In regions near the poles the opposite is true—
less solar energy is received than is lost.
The atmosphere balances
these differences by acting as a giant heat-transfer system. This
system moves warm air toward high latitudes and cool air toward
the equator. On a smaller scale, but for the same reason, ocean currents
also contribute to this global heat transfer. Global circulation is very
complex, but you can begin to understand it by first thinking about
circulation that would occur on a non-rotating Earth.
Global Winds
Use Visuals
L1
Figure 9 After students have examined
the illustration, ask: What happens to
surface air at the equator? (It rises
upward as it is warmed by the sun.) Why
does air flow from the poles to the
equator? (Air at the equator receives
more heat from the sun, thus making the
area lower in density and pressure than
colder air at the poles. Since higher
pressure air moves toward lower pressure
air, the net flow is toward the equator.)
Visual, Logical
How does the atmosphere
balance the unequal heating
of Earth’s surface?
Cold
flow
Surface
Conve
ll
ce
ion
Hot
flow
ce
fa
ur
e ct
nv
Co
Relating Cause and Effect Explain to
students that, as warm air rises from the
surface at the equator, cooler air coming
from the poles moves in to fill the space.
Ask: Why is the warm air in the upper
atmosphere above the equator drawn
toward the poles? (The tropopause
prevents the air from rising any higher.
The only possible direction of motion is
toward the poles.)
Verbal, Logical
ctio
n
ce
ll
Non-Rotating Earth Model On a
S
Build Science Skills
L2
Cold
Figure 9 Circulation on a
Non-Rotating Earth A simple
convection system is produced
by unequal heating of the
atmosphere.
Relating Cause and Effect
Why would air sink after reaching
the poles?
hypothetical non-rotating planet with a
smooth surface of either all land or all water,
two large thermally produced cells would
form, as shown in Figure 9. The heated air
at the equator would rise until it reached the
tropopause—the boundary between the troposphere and the stratosphere. The
tropopause, acting like a lid, would deflect this
air toward the poles. Eventually, the upper-level
airflow would reach the poles, sink, spread out in
all directions at the surface, and move back toward
the equator. Once at the equator, it would be reheated
and begin its journey over again. This hypothetical circulation system has upper-level air flowing toward the pole and surface
air flowing toward the equator.
Rotating Earth Model If the effect of rotation were added to
the global circulation model, the two-cell convection system would
break down into smaller cells. Figure 10 illustrates the three pairs of
cells that would carry on the task of redistributing heat on Earth. The
polar and tropical cells retain the characteristics of the thermally generated convection described earlier. The nature of circulation at the
middle latitudes, however, is more complex.
Near the equator, rising air produces a pressure zone known as the
equatorial low—a region characterized by abundant precipitation. As
shown in Figure 10, the upper-level flow from the equatorial low
reaches 20 to 30 degrees, north or south latitude, and then sinks back
toward the surface. This sinking of air and its associated heating due
540 Chapter 19
Facts and Figures
George Hadley, an English meteorologist of the
eighteenth century, first proposed the simple
convection system pictured in Figure 9. Because
Earth rotates, however, meteorologists had to
develop a more complex global circulation
model. This model is pictured in Figure 10 and
has three cells on each side of the equator. The
Hadley cells, named after George Hadley and
also called tropical cells, are shown north and
540 Chapter 19
south of the equator. The next cell, which is
unlabeled on the diagram, is the mid-latitude
cell. It is also called a Ferrel cell after William
Ferrel, a nineteenth century American
meteorologist who helped explain atmospheric
circulation at mid-latitudes. The third type of
cell, also unlabeled, is the polar cell.
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Polar high
Subpolar
to compression produce hot, arid condilow
Polar cell
tions. The center of this zone of sinking
Ferrel
Polar easterlies
dry air is the subtropical high, which
cell
60°
encircles the globe near 30 degrees
north and south latitude. The
great deserts of Australia,
Polar front
30°
Hadley
Ferrel
Arabia, and the Sahara in
cell
cell
North Africa exist because
Westerlies
Su
btro
p
ical h
of the stable dry conditions
igh
associated with the subHadley cell
NE
tropical highs.
trade winds
0°
At the surface, airflow
moves outward from the
Equ
atoria
l low
center of the subtropical
Hadley cell
high. Some of the air travels
Hadley
toward the equator and is
cell
SE
deflected by the Coriolis effect,
trade winds
producing the trade winds. Trade
winds are two belts of winds that
Ferrel cell
Ferrel
blow almost constantly from easterly
cell
directions. The trade winds are located
between the subtropical highs and the equator.
Figure 10 Circulation on a
The remainder of the air travels toward the poles and is
Rotating Earth This model of
deflected, generating the prevailing westerlies of the middle latitudes.
global air circulation proposes
three pairs of cells.
The westerlies make up the dominant west-to-east motion of the
Interpreting Diagrams Describe
atmosphere that characterizes the regions on the poleward side of the
the patterns of air circulation at
subtropical highs. As the westerlies move toward the poles, they
the equatorial and subpolar lows.
encounter the cool polar easterlies in the region of the subpolar low.
The polar easterlies are winds that blow from the polar high toward
the subpolar low. These winds are not constant winds like the trade
winds. In the polar region, cold polar air sinks and spreads toward the
equator. The interaction of these warm and cool air masses produces
the stormy belt known as the polar front.
This simplified global circulation is dominated by four pressure
zones. The subtropical and polar highs are areas of dry subsiding (sinking) air that flows outward at the surface, producing the prevailing
winds. The low-pressure zones of the equatorial and subpolar regions
are associated with inward and upward airflow accompanied by clouds
and precipitation.
What is the polar front?
Air Pressure and Wind
Use Visuals
L1
Figure 10 After students have read
Rotating Earth Model and examined the
illustration, ask: What happens to warm
equatorial air that has risen into the
upper atmosphere? (It moves toward
the poles until it reaches latitudes of 20 or
30 degrees, then sinks downward.) What
kind of weather is characteristic of
regions around 20 to 30 degrees
latitude? (dry, hot) What factor is
primarily responsible for the trade
winds? (Coriolis effect) What factors
create the polar front? (meeting of the
warmer, subpolar westerlies with the
colder polar easterlies) What kind of
weather is characteristic of the polar
front? (stormy)
Visual, Logical
L2
Students may think that heat is “lost”
from air as it sinks toward Earth’s surface
in a high-pressure center. Explain to
students that the term adiabatic refers to
the cooling or warming of air caused
when air is allowed to expand or is
compressed, not because heat is added
to or subtracted from the system. In
other words, no heat enters or leaves
the system. Compression of air as it sinks
creates adiabatic heating. Expansion of
air as it rises creates adiabatic cooling.
The air continues to cool as it rises until
it reaches its dew point. Then condensation takes place and clouds begin to
form. Ask: How does adiabatic cooling
help explain why precipitation is
associated with low-pressure centers
but not high-pressure centers? (Rising
air in a low-pressure center becomes cool
enough for condensation. Sinking air
in a high-pressure system is heated
adiabatically, preventing condensation.)
Verbal
541
Answer to . . .
Figure 9 Air becomes more dense as
it cools, causing it to sink.
Figure 10 Both are zones where two
cells, or air masses, converge and air
rises, forming zones of low pressure.
The atmosphere
transfers heat by moving
warm air toward high latitudes and
cool air toward the equator.
the stormy belt where
subpolar westerlies and
polar easterlies meet
Air Pressure and Wind 541
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Build Science Skills
11
10
L2
60°
101
102
1
Evaluate
Understanding
L2
Have students write a paragraph
comparing and contrasting cyclonic and
anticyclonic winds. Have students write
a second paragraph explaining how
each of the following global winds is
formed: trade winds, westerlies, and
polar easterlies.
Reteach
1014
7
101
10
1020
1 01
Pacific
High
Bermuda
High
30°
6
102
0°
1005
1014
1017
1020
101
10
14
1020
1023
17
10
02
10
1008
1011
ITCZ
05
999
1017
1023
1011
7
1014
1017
1023
1026
102
0
1023
30°
1020
1017
1014
1017
1014
1008
1008
1005
1002
1002
60°
Figure 11 Average Surface
Pressure and Associated
Global Circulation for July.
The ITCZ line stands for the
Intertropical Convergence Zone.
1020
1017
1014
1011
999
120°
3 ASSESS
1
0
14
102
3
The only truly continuous pressure belt is the subpolar low in
the Southern Hemisphere. Here
the ocean is uninterrupted by
ITCZ
landmasses. At other latitudes,
particularly in the Northern
Hemisphere where landmasses
break up the ocean surface, large
seasonal temperature differences
disrupt the pressure pattern.
Large landmasses, particularly
Asia, become cold in the winter
when a seasonal high-pressure
60°
120°
system develops. From this highpressure system, surface airflow is directed off the land. In the summer,
landmasses are heated and develop low-pressure cells, which permit air
to flow onto the land as shown in Figure 11. These seasonal changes in
wind direction are known as the monsoons. During warm months, areas
such as India experience a flow of warm, water-laden air from the Indian
Ocean, which produces the rainy summer monsoon. The winter monsoon is dominated by dry continental air. A similar situation exists to a
lesser extent over North America.
08
10
1008
14
10
Applying Concepts Use a world map
to help students visualize the movement
of monsoons in south Asia. Have them
picture a high-pressure system above
the land that pushes air out to sea. Then
have them picture how heat from the
land during summer causes air to rise,
producing a low-pressure system that
pulls in moist air from the ocean.
Visual, Logical
10
Influence of Continents
11
10
Section 19.2 (continued)
0°
L1
Draw a globe on the board or use a
laminated world map. On the map,
draw in each type of global wind while
explaining its formation to students. Use
Figure 10 as a reference.
Section 19.2 Assessment
Reviewing Concepts
1.
2.
Solutions
8. 992 (⫺) to 1020 (⫹) millibars. In other
words, the map includes pressures a bit
less than 992 millibars and a bit more
than 1020 millibars. Isobar interval
equals 4 millibars.
3.
4.
5.
6.
Describe how winds blow around pressure
centers in the Northern Hemisphere.
Compare the air pressure for a cyclone
with an anticyclone.
How does friction control the net flow of
air around a cyclone and an anticyclone?
Describe how the atmosphere balances
the unequal heating of Earth’s surface.
What is the only truly continuous pressure
belt? Why is it continuous?
In general, what type of weather can you
expect if a low-pressure system is moving into
your area?
Critical Thinking
7. Identifying Cause and Effect What must
happen in the air above for divergence at the
surface to be maintained? What type of
pressure center accompanies surface
divergence?
8. Examine Figure 7. What is the
approximate range of barometric
pressure indicated by the isobars on
the map? What is the pressure interval
between adjacent isobars?
542 Chapter 19
Section 19.2 Assessment
1. In the Northern Hemisphere, winds blow
counterclockwise and inward around a low,
and clockwise and outward around a high.
2. In cyclones, air pressure decreases toward
the center of the cell. In anticyclones, air pressure increases toward the center of the cell.
3. The effect of friction is to cause a net flow
of air inward around a cyclone and a net flow
outward about an anticyclone.
542 Chapter 19
4. The atmosphere acts like a huge heattransfer system, transporting warm air from
the equator toward the poles and cold air
from the poles toward the equator.
5. The subpolar low in the Southern
Hemisphere; no landmasses break up
the pressure system.
6. cloud formation and precipitation
7. Convergence must occur aloft in order
for a surface divergence to be maintained.
A surface divergence is associated with a
high-pressure center.