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Section 19.1
19.1 Understanding Air Pressure
1 FOCUS
Section Objectives
19.1
19.2
19.3
19.4
Describe how air pressure is
exerted on objects.
Explain how changes in air
pressure affect the mercury
column of a barometer.
Identify the ultimate energy
source for wind.
Describe how the Coriolis effect
influences freely moving objects.
Key Concepts
Vocabulary
Describe how air pressure
is exerted on objects.
◆
What happens to the
mercury column of a
barometer when air
pressure changes?
◆
◆
◆
◆
air pressure
barometer
pressure gradient
Coriolis effect
jet stream
Reading Strategy
Identifying Main Ideas Copy the table
below. As you read, write the main ideas for
each topic.
Topic
Main Ideas
Air Pressure Defined
Air pressure is the weight
of air above. It is exerted
in all directions.
Measuring Air Pressure
a.
?
Factors Affecting Wind
b.
?
What is the ultimate
energy source for wind?
How does the Coriolis
effect influence freemoving objects?
Reading Focus
Build Vocabulary
L2
Visualize Explain to students that the
Coriolis effect is named after French
mathematician Gaspard Gustave de
Coriolis, who described the effect of
Earth’s rotation on the direction of winds
and ocean currents in 1835. Tell students
that they may remember the term more
easily if they create a mental picture.
Have them imagine looking down at
Earth from a high-altitude aircraft
traveling down a line of longitude,
watching the globe rotate beneath them
as they move from one pole to the other.
Reading Strategy
O
Figure 1 These palm trees in
Corpus Christi, Texas, are buffeted
by hurricane-force winds.
f the various elements of weather and climate, changes in air pressure are the least noticeable. When you listen to a weather report, you
probably focus on precipitation, temperature, and humidity. Most
people don’t wonder about air pressure. Although you might not perceive hour-to-hour and day-to-day variations in air pressure, they are
very important in producing changes in our weather. For example, variations in air pressure from place to place can generate winds like those
shown in Figure 1. The winds, in turn, bring change in temperature and
humidity. Air pressure is one of the basic weather elements and is an
important factor in weather forecasting. Air pressure is closely tied to
the other elements of weather in a cause-and-effect relationship.
Air Pressure Defined
L2
Air pressure is simply the pressure exerted by the weight of air above.
Average air pressure at sea level is about 1 kilogram per square centimeter. This pressure is roughly the same pressure that is produced by a
column of water 10 meters in height. You can calculate that the air pressure exerted on the top of a 50-centimeter-by-100-centimeter school desk
exceeds 5000 kilograms, which is about the mass of a 50-passenger school
bus. Why doesn’t the desk collapse under the weight of the air above it?
Air pressure is exerted in all directions—down, up, and sideways.
The air pressure pushing down on an object exactly balances the air
pressure pushing up on the object.
a. Air pressure is measured with
barometers, using the principle that
fluids will deform if the weight of air
is exerted on them. b. the pressure
gradient, Coriolis effect, and friction
2 INSTRUCT
Air Pressure Defined
What is average air pressure at sea level?
Measuring the Mass
of Air
L2
Purpose Students prove air has mass
by comparing the mass of a balloon
when empty and when filled with air.
Materials balloon, balance
Procedure Blow up a balloon and
determine its mass using a balance.
Remove the balloon, release the air, and
replace the balloon on the balance. Find
the mass again.
Expected Outcome Students will see
that the empty balloon is lighter than
the air-filled balloon.
Visual, Logical
532 Chapter 19
532 Chapter 19
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Imagine a tall aquarium that has the same dimensions as the
desktop in the previous example. When this aquarium is filled to
a height of 10 meters, the water pressure at the bottom equals
1 atmosphere, or 1 kilogram per square centimeter. Now imagine
what will happen if this aquarium is placed on top of a student
desk so that all the force is directed downward. The desk collapses
because the pressure downward is greater than the pressure exerted
in the other directions. When the desk is placed inside the aquarium and allowed to sink to the bottom, however, the desk does not
collapse in the water because the water pressure is exerted in all
directions, not just downward. The desk, like your body, is built to
withstand the pressure of 1 atmosphere.
A
L1
Vacuum
Use Visuals
76 cm (29.92 in.)
Figure 2 After students have examined
both illustrations, have them explain in
their own words how each type of
barometer works. (Mercury barometer:
Air pressure pushes down on pool of
mercury, forcing the fluid to rise in the
tube. Aneroid barometer: Chamber with
air removed is made of a flexible material
that changes shape as air pressure
changes. Change in shape is transferred
to graph paper by a pen attached to the
outside of the chamber.)
Visual, Verbal
Mercury
Air
pressure
Measuring Air
Pressure
Air
pressure
Measuring Air Pressure
When meteorologists measure atmospheric pressure, they use a
Mercury
unit called the millibar. Standard sea-level pressure is 1013.2 millibars. You might have heard the phrase “inches of mercury,” which
Rotating
is used by the media to describe atmospheric pressure. This
cylinder
B
expression dates from 1643, when Torricelli, a student of the
famous Italian scientist Galileo, invented the mercury barometer.
A barometer is a device used for measuring air pressure
Chamber is
(bar ⫽ pressure, metron ⫽ measuring instrument).
squeezed
as air
Torricelli correctly described the atmosphere as a vast ocean
pressure
increases
of air that exerts pressure on us and all objects around us. To
measure this force, he filled a glass tube, closed at one end, with
mercury. He then put the tube upside down into a dish of mercury, as shown in Figure 2A. The mercury flowed out of the tube
Pen moves up and down
until the weight of the column was balanced by the pressure that
with pressure changes
the atmosphere exerted on the surface of the mercury in the
dish. In other words, the weight of mercury in the column (tube)
Figure 2 A Mercury Barometer
equaled the weight of the same size column of air that extended
Standard atmospheric pressure at
sea level is 29.92 inches of
from the ground to the top of the atmosphere.
mercury. B Aneroid Barometer
When air pressure increases, the mercury in the tube rises.
The recording mechanism
provides a continuous record of
When air pressure decreases, so does the height of the mercury
pressure changes over time.
column. With some improvements, the mercury barometer is still the
Applying Concepts Why would
standard instrument used today for measuring air pressure.
a continuous record help weather
forecasters?
The need for a smaller and more portable instrument for measuring air pressure led to the development of the aneroid barometer. The
aneroid barometer uses a metal chamber with some air removed. This
partially emptied chamber is extremely sensitive to variations in air
pressure. It changes shape and compresses as the air pressure increases,
and it expands as the pressure decreases. One advantage of the aneroid
barometer is that it can be easily connected to a recording device,
For: Links on atmospheric pressure
shown in Figure 2B. The device provides a continuous record of presVisit: www.SciLinks.org
sure changes with the passage of time.
Web Code: cjn-6191
Air Pressure and Wind
533
Air Pressure
L2
Purpose Students show that air pressure
is exerted in all directions—down, up,
and sideways.
Materials glass, water, cardboard, sink
or basin
Procedure Fill the glass with water to
the brim. Place a piece of cardboard on
top of the glass. Working over a sink or
basin, hold the cardboard in place and
invert the glass. Then, remove your
hand from the cardboard.
Expected Outcome The pressure of
air pushing up on the cardboard holds
it against the full glass of water,
preventing it from spilling.
Visual, Logical
Download a worksheet on
atmospheric pressure for students
to complete, and find additional
teacher support from NSTA
SciLinks.
Customize for English Language Learners
Explain to students that gradient comes from
the word grade. Have students discuss the
various definitions of the word grade and write
a sentence using each meaning. (to arrange in
a scale or series, as in to assign a grade; to level
off to a smooth surface, as in grading a building
site prior to construction; the degree of slope in a
road or other surface)
Answer to . . .
Figure 2 A continuous record would
show that patterns of pressure change
over time. This would help in correlating pressure changes with weather.
Average air pressure
at sea level is about
1 kilogram per square centimeter.
Air Pressure and Wind 533
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Section 19.1 (continued)
Factors Affecting Wind
Factors Affecting
Wind
Use Visuals
Q What is the lowest barometric
L1
Figure 3 Have students examine the
isobar lines on the weather map. Ask:
What symbol signifies calm air?
Where is this symbol located on this
map? (two concentric circles; away from
the high and low pressure cells) How do
the wind flag symbols signify wind
speed? (The number and length of lines
on the flag increase with wind speed;
pennant shapes are used in place of
multiple lines at speeds of 55 mph and
greater.) What does the circle at the
base of each wind flag indicate? (the
location where the wind measurement was
taken)
Visual, Logical
Build Reading Literacy
As important as vertical motion is, far more air moves horizontally,
the phenomenon we call wind. What causes wind?
Wind is the result of horizontal differences in air pressure. Air
flows from areas of higher pressure to areas of lower pressure. You may
have experienced this flow of air when opening a vacuum-packed can of
coffee or tennis balls. The noise you hear is caused by air rushing from the
higher pressure outside the can to the lower pressure inside. Wind is
nature’s way of balancing such inequalities in air pressure.
The
unequal heating of Earth’s surface generates pressure differences. Solar
radiation is the ultimate energy source for most wind.
If Earth did not rotate, and if there were no friction between moving
air and Earth’s surface, air would flow in a straight line from areas of
higher pressure to areas of lower pressure. But both factors do exist so
the flow of air is not that simple.
Three factors combine to control wind: pressure differences, the Coriolis effect, and friction.
L1
Refer to p. 530D, which provides the
guidelines for making inferences.
barometric pressures have been
associated with strong hurricanes. The record for the
United States is 888 millibars
(26.20 inches) measured during
Hurricane Gilbert in September
1988. The world’s record,
870 millibars (25.70 inches),
occurred during Typhoon Tip,
a Pacific hurricane, in October
1979. Although tornadoes
undoubtedly have produced
even lower pressures, they have
not been accurately measured.
Pressure Differences Wind is created from differences in pressure—the greater these differences are, the greater the wind speed is. Over
Earth’s surface, variations in air pressure are determined from barometric readings taken at hundreds of weather stations. These pressure data are
shown on a weather map, like the one in Figure 3, using isobars. Isobars
are lines on a map that connect places of equal air pressure. The spacing
of isobars indicates the amount of pressure change occurring over a given
distance. These pressure changes are expressed as the pressure gradient.
Figure 3 Isobars The
distribution of air pressure is
shown on weather maps using
isobar lines. Wind flags indicate
wind speed and direction. Winds
blow toward the station circles.
Interpreting Visuals Use the
data on this map to explain
which pressure cell, high or low,
has the fastest wind speeds.
1020
1024
1028
1024
1020
1016
1016
1020
ff
H
16
10
1012
Miles
per hour
Calm
1–2
3–8
1008
9–14
21–25
4
10
00
996
15–20
100
16
10
Make Inferences Review the kinetic
theory of matter, which states that the
particles of a gas are always in motion.
Ask: How does the kinetic theory of
matter explain why atmospheric
gases move from areas of higher
pressure to areas of lower pressure?
(The higher the pressure, the more dense
the gas, which means the particles of the
gas are closer together and collide with
one another more often. Collisions add to
the motion of the particles, gradually
moving them out into areas of lower
pressure and density, where collisions
become less frequent.)
Verbal, Logical
pressure ever recorded?
A All of the lowest recorded
26–31
L
32–37
38–43
10
20
44–49
50–54
55–60
61–66
102
4
67–71
72–77
78–83
84–89
1024 1020
1016
1016
1020
1024
119–123
534 Chapter 19
Facts and Figures
The only force acting on a stationary parcel of
air is the pressure-gradient force. Once the air
begins to accelerate, the Coriolis effect deflects
it—to the right in the Northern Hemisphere,
to the left in the Southern Hemisphere.
Greater wind speeds result in a stronger
Coriolis effect, therefore a stronger deflection,
until the pressure-gradient force is equal and
opposite to the force of the Coriolis effect. At
534 Chapter 19
this point the wind direction is parallel to the
isobars and the flow is called a geostrophic
wind. Geostrophic winds occur only where
isobars are straight and evenly spaced, and
where there are no frictional forces. They are
rare in nature, though airflow in the upper
troposphere can sometimes closely approach
geostrophic winds.
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Integrate Physics
A steep pressure gradient, like a steep hill, causes greater acceleration of a parcel of air. A less steep pressure gradient causes a slower
acceleration.
Closely spaced isobars indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure
gradient and light winds. The pressure gradient is the driving force of
wind. The pressure gradient has both magnitude and direction. Its magnitude is reflected in the spacing of isobars. The direction of force is
always from areas of higher pressure to areas of lower pressure and at
right angles to the isobars. Friction affects wind speed and direction.
The Coriolis effect affects wind direction only.
Coriolis Effect The weather map in Figure 3 shows typical air
movements associated with high- and low-pressure systems. Air moves
out of the regions of higher pressure and into the regions of lower pressure. However, the wind does not cross the isobars at right angles as
you would expect based solely on the pressure gradient. This change in
movement results from Earth’s rotation and has been named the
Coriolis effect.
The Coriolis effect describes
North
Pole
how Earth’s rotation affects moving
objects. All free-moving objects or
fluids, including the wind, are
deflected to the right of their path of
motion in the Northern Hemisphere.
Target
In the Southern Hemisphere, they
Equator
150° 135° 120° 105° 90° 75° 60°
are deflected to the left. The reason
for this deflection is illustrated in
Figure 4. Imagine the path of a rocket Rotating Earth
Rotation Target
launched from the North Pole toward
a target located on the equator. The
Figure 4 The Coriolis Effect
true path of this rocket is straight, and the path would appear to be
Because Earth rotates 15° each
straight to someone out in space looking down at Earth. However, to
hour, the rocket’s path is curved
and veers to the right from the
someone standing on Earth, it would look as if the rocket swerved off its
North Pole to the equator.
path and landed 15 degrees to the west of its target.
Calculating How many degrees
This slight change in direction happens because Earth would have
does Earth rotate in one day?
rotated 15 degrees to the east under the rocket during a one-hour flight.
The counterclockwise rotation of the Northern Hemisphere causes path
deflection to the right. In the Southern Hemisphere, the clockwise rotation produces a similar deflection, but to the left of the path of motion.
The apparent shift in wind direction is attributed to the Coriolis
effect. This deflection: 1) is always directed at right angles to the direction
of airflow; 2) affects only wind direction and not wind speed; 3) is affected
by wind speed—the stronger the wind, the greater the deflection; and
4) is strongest at the poles and weakens toward the equator, becoming
nonexistent at the equator.
Air Pressure and Wind
L2
Gas Laws Review with students the laws
that govern the behavior of enclosed
gases. Charles’s law states that the
volume of a gas is directly proportional
to its temperature, assuming constant
pressure and density. Boyle’s law states
that the volume of a gas is inversely
proportional to the pressure, assuming
constant temperature and density.
The combined gas law describes
the interrelationships of temperature,
pressure, volume, and density in gases.
Ask: In what way do the gas laws,
which are stated in terms of enclosed
gases, apply to gases in the
atmosphere? (Gases in the atmosphere
are not in a closed system. However, the
relationship between temperature and
pressure, as described by the gas laws, still
applies. As temperature increases, pressure
decreases. A decrease in pressure also
decreases the density of the gases. Density
differences help explain the movement
of air masses. The direction of motion of
a mass of atmospheric gas at one
temperature and pressure is influenced
by other, nearby masses of different
temperatures and pressures. For example,
a mass of high-pressure, dense air will
tend to move into an area of lowerpressure, less dense air—either up into the
less dense regions of the atmosphere or
into a neighboring area of low pressure.)
Verbal, Logical
535
Facts and Figures
Over very short distances, the Coriolis effect is
too small to be noticed. Nevertheless, in the
middle latitudes the Coriolis effect is great
enough to potentially affect the outcome of a
baseball game. A ball hit a horizontal distance
of 100 m in 4 s down the right field line will
be deflected 1.5 cm to the right by the
Coriolis effect. This could be just enough to
turn a potential home run into a foul ball.
Answer to . . .
Figure 3 For this particular map,
isobar lines are most closely spaced in
the low-pressure cell, which indicates
fastest wind speeds in this cell.
Figure 4 360°
Air Pressure and Wind 535
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Section 19.1 (continued)
Build Science Skills
Effect of Friction
L2
Comparing and Contrasting Help
students distinguish the effects of friction
from the effects of the Coriolis force on
wind direction. Point out that in the
upper atmosphere, only the Coriolis
effect and pressure gradients determine
wind direction. At altitudes up to about
600 m, wind speed and direction are
affected by those two factors plus
friction. Ask: How does friction interact
with the Coriolis effect? (The Coriolis
effect increases with wind speed. Friction
decreases wind speed, which in turn
decreases the directional influence of the
Coriolis effect at lower altitudes.)
Verbal, Logical
3 ASSESS
Evaluate
Understanding
L2
Have students write a paragraph
describing how air pressure gradients
create wind and how the Coriolis effect
and friction influence wind speed and/or
direction. (Dense air in areas of higher
pressure move toward less dense air in
areas of lower pressure. This flow of air is
the wind. The Coriolis effect deflects wind
motion relative to the ground because of
Earth’s rotation, but does not affect wind
speed. Friction between air and the ground
reduces wind speed at low altitudes.)
Reteach
L1
Have students work in pairs or small
groups to explain to each other how air
pressure gradients form and how they
create wind. Students should also explain
how the Coriolis effect influences wind
direction and how friction influences wind
speed and direction. Listen to student
explanations and offer corrections as
needed.
Unequal heating of Earth’s surface by
solar radiation results from differences
in angles of incident radiation and
differential heating of land and water.
Unequal heating produces air pressure
differences that in turn drive global
circulation of air.
536 Chapter 19
Friction The effect of friction on wind is important
only within a few kilometers of Earth’s surface. Friction
acts to slow air movement, which changes wind direcLow
tion. To illustrate friction’s effect on wind direction,
Pressure
first think about a situation in which friction does not
gradient force
play a role in wind’s direction.
Wind
When air is above the friction layer, the pressure gradient
causes air to move across the isobars. As soon as air
Coriolis effect
starts to move, the Coriolis effect acts at right angles to
High
this motion. The faster the wind speed, the greater the
A
deflection is. The pressure gradient and Coriolis effect
Low
balance in high-altitude air, and wind generally flows
parallel to isobars, as shown in Figure 5A. The most
Pressure
gradient force
prominent features of airflow high above the friction
Wind
layer are the jet streams. Jet streams are fast-moving
rivers of air that travel between 120 and 240 kilometers
Friction
Coriolis effect
per hour in a west-to-east direction. One such jet stream
High
is situated over the polar front, which is the zone separating the cool polar air from warm subtropical air. Jet
B
streams originally were encountered by high-flying
bombers during World War II.
Figure 5 A Upper-level wind
For air close to Earth’s surface, the roughness of the terrain deterflow is balanced by the Coriolis
mines the angle of airflow across the isobars. Over the smooth ocean
effect and pressure gradient
forces. B Friction causes surface
surface, friction is low, and the angle of airflow is small. Over rugged
winds to cross isobars and move
terrain, where the friction is higher, winds move more slowly and cross
toward lower pressure areas.
the isobars at greater angles. As shown in Figure 5B, friction causes
wind to flow across the isobars at angles as great as 45 degrees. Slower
wind speeds caused by friction decrease the Coriolis effect.
Section 19.1 Assessment
Reviewing Concepts
1.
2.
3.
4.
5.
Why don’t objects such as a table collapse
under the weight of air above them?
Suppose the height of a column in a
mercury barometer is decreasing. What is
happening?
What is the ultimate energy source for
most wind?
How does the Coriolis effect influence
motion of free-moving objects?
Why do jet streams flow parallel to isobars?
Critical Thinking
6. Interpreting illustrations Study Figures 5A
and 5B. Why are the wind arrows drawn to
different lengths in these figures?
Solar Radiation Review section 17.3.
Describe examples of unequal heating of
Earth’s atmosphere that could lead to air
pressure differences that ultimately influence wind.
536 Chapter 19
Section 19.1 Assessment
1. because air pressure is exerted in all
directions
2. Air pressure is decreasing.
3. the sun
4. The Coriolis effect deflects free-moving
objects to the right (in the Northern
Hemisphere) or to the left (in the
Southern Hemisphere).
5. Because jet streams are generally above the
friction layer, the effect of friction is negligible.
Jet stream wind direction is balanced by the
pressure gradient and the Coriolis effect. The
net effect of the balance is for winds to flow
parallel to isobars.
6. Different lengths represent different magnitudes; the longer the arrow, the faster the
wind speed.
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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.
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Section 19.3
19.3 Regional Wind Systems
1 FOCUS
Section Objectives
Key Concepts
Vocabulary
What causes local winds?
◆
Describe the general
movement of weather in
the United States.
◆
◆
prevailing wind
anemometer
El Niño
What happens when
unusually strong, warm
ocean currents flow along
the coasts of Ecuador and
Peru?
How is a La Niña event
triggered?
Reading Strategy
19.9
Previewing Copy the table below. Before
you read, use Figure 17 to locate examples of
the driest and wettest regions on Earth. After
you read, identify the dominant wind system
for each location.
Precipitation
Location
Extremely low
Extremely high
Dominant
Wind System
a.
?
b.
?
c.
?
d.
?
19.10
19.11
Identify the causes of local
winds.
Describe the general movement
of weather in the United States.
Compare and contrast weather
patterns characteristic of El
Niño and La Niña events.
Reading Focus
Build Vocabulary
Word Parts Explain to students that
the word anemometer is a combination
of the Greek word anemos, meaning
“wind,” and the Latin meter. Have
students use this information to predict
the definition of anemometer. After they
have studied the section, have them
correct their predictions if necessary.
C
irculation in the middle latitudes is complex and does not fit the
convection system described for the tropics. Between about 30 and 60
degrees latitude, the general west-to-east flow, known as the westerlies, is interrupted by migrating cyclones and anticyclones. In the
Northern Hemisphere, these pressure cells move from west to east
around the globe.
Local Winds
Small-scale winds produced by a locally generated pressure gradient
are known as local winds.
The local winds are caused either by
topographic effects or by variations in surface composition—land
and water—in the immediate area.
Land and Sea Breezes In coastal areas during the
warm summer months, the land surface is heated more
intensely during the daylight hours than an adjacent body of
water is heated. As a result, the air above the land surface
heats, expands, and rises, creating an area of lower pressure.
As shown in Figure 12, a sea breeze then develops because
cooler air over the water at higher pressure moves toward the
warmer land and low pressure air. The breeze starts developing shortly before noon and generally reaches its greatest
intensity during the mid- to late afternoon. These relatively
cool winds can be a moderating influence on afternoon temperatures in coastal areas.
Reading Strategy
Figure 12 Sea Breeze During
daylight hours, the air above land
heats and rises, creating a local
zone of lower air pressure.
988 mb
992 mb
L
H
1004 mb
1008 mb
Cool water
H
L
Warm land
Air Pressure and Wind
543
L2
Sample answers:
a. Arabian Desert
b. sinking trade winds produce tropical
high
c. Amazon rain forest
d. rising trade winds produce equatorial
low
Local Winds
Use Visuals
996 mb
1000 mb
1016 mb
L2
L1
Figure 12 After students have examined
the illustration, ask: Does a sea breeze
move toward the sea or away from
the sea? (away from the sea) Why does
a sea breeze usually reach its highest
speeds in the late afternoon? (The
heat differential between land and water
increases with the number of hours the
sun has been shining.)
Visual, Logical
Air Pressure and Wind 543
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Section 19.3 (continued)
L1
Use Visuals
988 mb
H
Figure 13 After students have examined
the illustration, ask: Does a land breeze
move toward the land or away from
the land? (away from the land) Why do
land breezes usually take place at
night? (After sunset, both land and water
begin to cool, but land cools more rapidly.)
Visual, Logical
996 mb
1004 mb
1008 mb
L
Day/
Night
toward
land
day
sun heats land
faster than
water; air above
land rises,
creating low
pressure zone
land
toward
sea
night
land cools
faster than
water; air
above land
sinks, creating
high pressure
zone relative to
air above water
valley
upslope
mountain downslope
day
night
air along slopes
heats faster
than air above
valley, glides
up slope
because it is
less dense
Warm air
A
Figure 14 A Valley Breeze
Heating during the day generates
warm air that rises from the
valley floor. B Mountain Breeze
After sunset, cooling of the air
near mountain slopes can result in
cool air moving into the valley.
air along
slopes cools
faster than air
above valley,
sinks down
slope into
valley
1016 mb
Cool land
Figure 13 Land Breeze At night,
the land cools more rapidly than
the sea, generating an offshore
flow called a land breeze.
Inferring How would the isobar
lines be oriented if there was no
air pressure change across the
land–water boundary?
Cause
sea
H
Warm water
Comparing and Contrasting Have
students create a table that contrasts sea,
land, valley, and mountain breezes.
Direction
992 mb
1000 mb
L2
Build Science Skills
L
At night, the reverse may take place. The land cools more
rapidly than the sea, and a land breeze develops, as shown in
Figure 13. The cooler air at higher pressures over the land
moves to the sea, where the air is warmer and at lower pressures. Small-scale sea breezes also can develop along the
shores of large lakes. People who live in a city near the Great
Lakes, such as Chicago, recognize this lake effect, especially
in the summer. They are reminded daily by weather reports
of the cool temperatures near the lake as compared to
warmer outlying areas.
Valley and Mountain Breezes A daily wind similar to land and sea breezes occurs in many mountainous
regions. During daylight hours, the air along the slopes of the mountains is heated more intensely than the air at the same elevation over
the valley floor. Because this warmer air on the mountain slopes is less
dense, it glides up along the slope and generates a valley breeze, as
shown in Figure 14A. The occurrence of these daytime upslope breezes
can often be identified by the cumulus clouds that develop on adjacent mountain peaks.
After sunset, the
pattern may reverse.
The rapid cooling of
the air along the
mountain slopes produces a layer of cooler
air next to the ground.
Because cool air is
denser than warm air,
Cool air
it moves downslope
B
into the valley. Such a
movement of air,
illustrated in Figure 14B, is called a mountain breeze. In the Grand
Canyon at night, the sound of cold air rushing down the sides of the
canyon can be louder than the sound of the Colorado River below.
The same type of cool air drainage can occur in places that have
very modest slopes. The result is that the coldest pockets of air are usually found in the lowest spots. Like many other winds, mountain and
valley breezes have seasonal preferences. Although valley breezes are
most common during the warm season when solar heating is most
intense, mountain breezes tend to be more dominant in the cold season.
What type of local wind can form in the Grand
Canyon at night?
Verbal, Logical
Differential Heating
L2
544 Chapter 19
Purpose Students observe that soil heats
faster than water.
Materials 2 containers, soil, water,
2 thermometers
Customize for English Language Learners
Procedure Fill the two containers 2/3
full, one with water and the other with
soil. Place a thermometer in each
container and place both in a hot, sunny
location. Read temperature measurements
at the beginning of the experiment and
after about 25 minutes.
Explain that the terms prevailing and predominant are synonyms. They are adjectives
that mean “having superior strength, influence,
or authority.” Have students look up the
definitions of verb forms of these words and
use each in a sentence. Then ask students to
Expected Outcome Soil heats faster
than water.
Visual, Kinesthetic
544 Chapter 19
look up definitions for the related verb
dominate and the related adjective dominant.
Students also could be asked to compare and
contrast the meanings of all six words, as well
as the suffix pre-.
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How Wind Is
Measured
How Wind Is
Measured
Two basic wind measurements—
direction and speed—are particularly
important to the weather observer.
Winds are always labeled by the direction from which they blow. A north
wind blows from the north toward the
south. An east wind blows from the
east toward the west. The instrument
most commonly used to determine
wind direction is the wind vane,
shown in the upper right of Figure 15.
Wind vanes commonly are located on
buildings, and they always point into
the wind. The wind direction is often
shown on a dial connected to the
wind vane. The dial indicates wind
direction, either by points of the compass—N, NE, E, SE, etc.—or by a scale
of 0° to 360°. On the degree scale, 0°
or 360° are north, 90° is east, 180° is
south, and 270° is west.
Toward which direction does a SE wind blow?
L1
Use Visuals
Figure 15 After students have examined
the photograph, ask: What is the
purpose of the cup-shaped side of
the device shown in the photograph?
(to measure wind speed) What is the
purpose of the opposite end of the
device? (to indicate wind direction) What
might be the purpose of the boxshaped component on the pole
beneath the anemometer and wind
vane? (could contain devices for recording
wind speed and direction data, and/or
transmitting the data to another location)
Visual, Logical
Use Community
Resources
Figure 15 Wind Vane and Cup
Anemometer
Interpreting Photographs How
can you tell which direction the
wind is blowing?
Wind Direction When the wind consistently blows more often
from one direction than from any other, it is called a prevailing wind.
Recall the prevailing westerlies that dominate circulation in the middle
latitudes.
In the United States, the westerlies consistently move
weather from west to east across the continent. Along within this general eastward flow are cells of high and low pressure with the
characteristic clockwise and counterclockwise flows. As a result, the
winds associated with the westerlies, as measured at the surface, often
vary considerably from day to day and from place to place. In contrast,
the direction of airflow associated with the trade winds is much more
consistent.
L2
Take students on a field trip to visit
a weather station maintained by a
community member or local or government organization. Ask the person in
charge to show students how wind
direction and speed are measured at
the station, how records are kept,
and how the information is used.
Interpersonal, Kinesthetic
Wind Speed Shown in the upper left of Figure 15, a cup anemometer (anemo ⫽ wind, metron ⫽ measuring instrument) is commonly used
to measure wind speed. The wind speed is read from a dial much like the
speedometer of an automobile. Places where winds are steady and speeds
are relatively high are potential sites for tapping wind energy.
For: Links on winds
Visit: www.SciLinks.org
Web Code: cjn-6193
Air Pressure and Wind
545
Download a worksheet on winds
for students to complete, and find
additional teacher support from
NSTA SciLinks.
Answer to . . .
Figure 13 Isobars would be
horizontal.
Figure 15 Wind vanes point into
the wind.
mountain breeze
NW
Air Pressure and Wind 545
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Section 19.3 (continued)
Polar jet
El Niño and La Niña
Build Reading Literacy
North
America
Asia
L1
Subtropical jet
Refer to p. 246D in Chapter 9, which
provides the guidelines for relating
cause and effect.
Relate Cause and Effect As they read
through the text on El Niño and La Niña,
have students create a flowchart showing
the cause-effect chain that results in an
El Niño event. (Sample answer: high
pressure over eastern Pacific → warm
water pileup in eastern Pacific that
prevents cold water upwelling and
promotes formation of low air pressure
zone → inland areas receive abnormal
amounts of rain → trade winds weaken or
may reverse direction → subtropical and
mid-latitude jet streams are displaced →
abnormal rainfall in Florida/southern
United States and mild winter temperatures
west of the Rocky Mountains)
Verbal, Logical
Warm
water
Low
pressure
Strong trade winds
Cool
water
Equatorial currents (strong)
Ecuador
Peru
South
America
High
pressure
Australia
Figure 16 Normal Conditions
Trade winds and strong
equatorial ocean currents flow
toward the west.
Strong
Peruvian
current
El Niño and La Niña
Look at Figure 16. The cold Peruvian current flows toward the equator along the coasts of Ecuador and Peru. This flow encourages
upwelling of cold nutrient-filled waters that are the primary food
source for millions of fish, particularly anchovies. Near the end of the
year, however, a warm current that flows southward along the coasts of
Ecuador and Peru replaces the cold Peruvian current. During the nineteenth century, the local residents named this warm current El Niño
(“the child”) after the Christ child because it usually appeared during
the Christmas season. Normally, these warm countercurrents last for
a few weeks and then give way to the cold Peruvian flow again.
El Niño
At irregular intervals of three to seven years, these
warm countercurrents become unusually strong and replace normally cold offshore waters with warm equatorial waters. Scientists
use the term El Niño for these episodes of ocean warming that affect
the eastern tropical Pacific.
The onset of El Niño is marked by abnormal weather patterns that
drastically affect the economies of Ecuador and Peru. As shown in
Figure 17, these unusually strong undercurrents accumulate large quantities of warm water that block the upwelling of colder, nutrient-filled
water. As a result, the anchovies starve, devastating the local fishing
industry. At the same time, some inland areas that are normally arid
receive an abnormal amount of rain. Here, pastures and cotton fields
have yields far above the average. These climatic fluctuations have been
known for years, but they were originally considered local phenomena.
It now is understood that El Niño is part of the global circulation and
that it affects the weather at great distances from Peru and Ecuador.
When an El Niño began in the summer of 1997, forecasters predicted that the pool of warm water over the Pacific would displace the
546 Chapter 19
Facts and Figures
The seesaw pattern of atmospheric pressure
changes between the eastern and western
Pacific is called the Southern Oscillation.
During average years, high pressure over the
eastern Pacific causes surface winds and warm
equatorial waters to flow westward. The result
is a pileup of warm water in the western
Pacific, which promotes the lowering of air
546 Chapter 19
pressure. An El Niño event begins as surface
pressure increases in the western Pacific and
decreases in the eastern Pacific. This air
pressure reversal weakens or may even reverse
the trade winds, and results in an eastward
movement of the warm waters that had
accumulated in the western Pacific.
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Integrate Physics
Warmer than
average winter
Asia
Polar jet
North
America
Wetter than
average winter
Subtropical jet
Weak trade winds
Pressure
increases
Ecuador
Strong counter current
Dryer
than
average
Warm
water
Peru
South
America
Australia
Pressure
decreases
paths of both the subtropical and midlatitude jet streams, as shown in
Figure 17. The jet streams steer weather systems across North America.
As predicted, the subtropical jet brought rain to the Gulf Coast. Tampa,
Florida, received more than three times its normal winter precipitation. The mid-latitude jet pumped warm air far north into the
continent. As a result, winter temperatures west of the Rocky
Mountains were significantly above normal.
Weak
Peruvian
current
Figure 17 El Niño Warm
countercurrents cause reversal of
pressure patterns in the western
and eastern Pacific.
L2
Thermal Energy Transfer Remind
students that heat is the transfer of
energy between two objects of different
temperatures. Heat can be transferred
directly, when the objects are in contact.
It also can be transferred indirectly, by
radiation and convection. Radiation is
the transfer of energy via electromagnetic waves. Convection is the
transfer of energy by fluid motion. In a
fluid, convection works when colder,
denser fluid sinks below warmer, less
dense fluid. Ask: Which of these three
types of heat transfer is involved in El
Niño formation and the weather
patterns that result? (All three; direct
transfer takes place between ocean water
and air. Radiation transfers heat from sun
to land and atmosphere. Convection is the
airflow that occurs between high- and lowpressure regions.)
Verbal, Logical
What is an El Niño and what effect does it have on
weather?
La Niña The opposite of El Niño is an atmospheric phenomenon
known as La Niña. Once thought to be the normal conditions that
occur between two El Niño events, meteorologists now consider
La Niña an important atmospheric phenomenon in its own right.
Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event
is triggered that has a distinctive set of weather patterns. A typical La
Niña winter blows colder than normal air over the Pacific Northwest
and the northern Great Plains. At the same time, it warms much of the
rest of the United States. The Northwest also experiences greater precipitation during this time. During the La Niña winter of 1998–99, a
world-record snowfall for one season occurred in Washington State. La
Niña impact can also increase hurricane activity. A recent study concluded that the cost of hurricane damages in the United States is 20
times greater in La Niña years as compared to El Niño years.
The effects of both El Niño and La Niña on world climate are widespread and vary greatly. These phenomena remind us that the air and
ocean conditions of the tropical Pacific influence the state of weather
almost everywhere.
For: Links on La Niña and El Niño
Visit: www.SciLinks.org
Web Code: cjn-6211
Air Pressure and Wind
547
Download a worksheet on La Niña
and El Niño for students to
complete, and find additional
teacher support from NSTA
SciLinks.
Facts and Figures
When winds blow along the surface of the
ocean, the water is not pushed along directly
in front of the wind. Instead, it is deflected by
about 45 degrees as a result of the Coriolis
effect. When surface waters are deflected,
colder, deeper water rises to take its place.
This cold water is richer in nutrients than
warmer surface water and promotes highly
productive coastal fisheries. In years when
coastal upwelling does not occur, coastal fish
populations may be reduced or absent.
Answer to . . .
An episode—occurring
every three to seven
years—of ocean warming that affects
the eastern tropical Pacific; warm
countercurrents become unusually
strong and replace normally cold
offshore waters with warm equatorial
waters.
Air Pressure and Wind 547
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Section 19.3 (continued)
Global Precipitation
Global Distribution
of Precipitation
Figure 18
Regions The map shows
Answers
Using the Map Key less than 400 mm
Identify Causes subtropical high
pressure cells
average annual precipitation
in millimeters. Using the
Map Key Determine the
range of precipitation that
dominates Northern Africa.
Identify Causes Which
weather pattern influences
precipitation in this area?
60°
30°
0°
30°
Tropic of Cancer
Equator
0°
Tropic of Capricorn
60°
30°
Precipitation in mm
< 400
400–800
800–1600
>1600
60°
Antarctic Circle
L2
To assess students’ knowledge of section
content, have them write a short
description of each of the four types of
local winds. Then have students write
two or three sentences describing the
events that give rise to an El Niño event.
Reteach
60°
30°
3 ASSESS
Evaluate
Understanding
Arctic Circle
Global Distribution of Precipitation
Figure 18 shows that the tropical region dominated by the equatorial
low is the rainiest region on Earth. It includes the rain forests of the
Amazon basin in South America and the Congo basin in Africa. In these
areas, the warm, humid trade winds converge to yield abundant rainfall throughout the year. In contrast, areas dominated by the subtropical
high-pressure cells are regions of extensive deserts. Variables other than
pressure and wind complicate the pattern. For example, the interiors
of large land masses commonly experience decreased precipitation.
However, you can explain a lot about global precipitation if you apply
your knowledge of global winds and pressure systems.
L1
Have students draw and label their own
diagrams illustrating sea, land, valley,
and mountain breezes. Allow students
to use Figures 12, 13, and 14 as
references, if needed.
Section 19.3 Assessment
Student paragraphs should contrast the
ocean water temperature conditions
that trigger each phenomenon and
describe how these in turn influence air
pressure. Specific examples of weather
associated with El Niño and La Niña also
should be included in the paragraph.
Reviewing Concepts
1.
2.
3.
4.
5.
What are local winds, and how are they
caused?
Describe the general movement of
weather in the United States.
What happens when strong, warm
countercurrents flow along the coasts of
Ecuador and Peru?
How is a La Niña event recognized?
What two factors mainly influence global
precipitation?
Critical Thinking
6. Interpreting illustrations Study Figure 17.
How could air pressure changes influence
weather patterns in this region?
Compare-Contrast Paragraph Write a
paragraph comparing the features and
effects of El Niño and La Niña. Include
specific weather patterns associated with
each phenomenon.
548 Chapter 19
Section 19.3 Assessment
1. Local winds are small-scale winds caused
by local variations in air pressure, which are
caused by topography and unequal heating
of land and water, or unequal heating of air
above slopes and valleys.
2. from west to east
3. An El Niño event blocks normal upwelling
of cold, nutrient-laden waters off the shores of
these countries, and causes heavy precipitation
inland. El Niño can have far-reaching effects
548 Chapter 19
on weather patterns of regions that are great
distances from Ecuador and Peru.
4. La Niña is triggered when surface temperatures in the eastern Pacific are colder than
average.
5. moisture content of air and distribution of
land and water
6. Warm water near Ecuador and Peru would
heat air and cause it to rise, producing lower
pressure and precipitation. Regions north of
Australia that normally receive more warm
water would experience higher pressure,
possibly leading to drought.
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Tracking El Niño from Space
The images in Figure 19 show the progression of the
1997–98 El Niño. They were derived from data collected by the satellite TOPEX/Poseidon.* This
satellite bounces radar signals off the ocean surface
to precisely measure the distance between the satellite and the sea surface. When combined with
high-precision data from the Global Positioning
System (GPS) of satellites, maps of sea-surface
topography like these can be produced. These maps
show the topography of the sea surface. The presence of hills indicates warmer-than-average water,
and the areas of low topography, or valleys, indicate
cooler-than-normal water. Using water topography,
scientists can determine the speed and direction of
surface ocean currents.
The colors in these images show sea-level height relative to the average. When you focus on the images,
remember that hills are warm colors and valleys are
cool colors. The white and red areas indicate places
of higher-than-normal sea-surface heights. In the
white areas, the sea surface is between 14 and 32
centimeters above normal. In the red areas, sea level
is elevated by about 10 centimeters. Green areas
indicate average conditions, whereas purple shows
zones that are at least 18 centimeters below average sea level.
The images show the progression of the large warmwater mass from west to east across the equatorial
Pacific Ocean. At its peak in November 1997, the
surface area covered by the warm water mass was
about one and one half times the size of the 48 contiguous United States. The amount of warm water
added to the eastern Pacific with a temperature
between 21°C and 30°C was about 30 times the
combined volume of the water in all of the United
States Great Lakes.
*Source: NASA’s Goddard Space Flight Center
April 25, 1997
July 25, 1997
November 10, 1997
March 14, 1998
Tracking El Niño
from Space
L2
Background
The image for April 25, 1997, shows
the Pacific Ocean on the eve of the
1997–1998 El Niño event. By examining
the images for July 25 and November
10, you can see the dramatic buildup of
warm water (white and red areas) in the
eastern Pacific and the enlarging region
of cooler water (purple) in the western
Pacific. By the time of the March 14,
1998, image, the area of warm water in
the eastern Pacific was much smaller.
Teaching Tips
• The directional terms eastern Pacific
and western Pacific may give some
students trouble, since the eastern
Pacific Ocean borders the western
coast of the United States. Ask: What
continents are bordered by the
eastern Pacific Ocean? (North
America, South America, and Central
America)
• As students examine each of the four
images in Figure 19, ask: What colors
indicate warmer than average water
temperatures? (white and red) What
colors indicate cooler than average
water temperatures? (purple) Where
is the warm water mass located on
April 25, 1997? (warm water mass not
yet evident) Where is the warm water
mass located on July 25 and
November 10? (eastern Pacific) What
does the map indicate about the
warm water mass on March 14?
(It is dissipating.)
Figure 19 Progression of the 1997–98 El Niño
Air Pressure and Wind
549
Air Pressure and Wind 549