24.3 Solar Energy and Winds

24.3 Solar Energy and Winds
Key Concepts
What happens to the
energy Earth receives
from the sun?
How is energy transferred
within the troposphere?
What causes winds?
What are some examples
of local winds and global
winds?
Vocabulary
◆ greenhouse effect
◆ wind
◆ local wind
◆ sea breeze
◆ land breeze
◆ global winds
◆ Coriolis effect
◆ monsoon
◆ jet stream
Reading Strategy
Comparing and Contrasting Copy the
table below. After you read, compare sea
and land breezes by completing the table.
Type of
Wind
Sea breeze
Land breeze
The heat you feel when you’re out in the sun is one effect of solar
energy. You might think that sunlight heats the air directly, just as it
heats you directly. But the process is more complicated than that.
Energy in the Atmosphere
Some solar energy that reaches Earth’s atmosphere is
reflected back, some is absorbed by the atmosphere, and
some is absorbed by Earth’s surface. About 30 percent of
the incoming solar energy is reflected back into space by
clouds, dust in the air, gases, and Earth’s surface. About 20
percent of the sun’s energy is absorbed by clouds and gases.
But the greatest amount of solar energy—about half—passes
through the atmosphere and is absorbed by the surface.
The atmosphere is heated primarily by energy that is
reradiated by Earth’s surface. Unlike incoming solar
energy, which has much of its energy in the visible spectrum,
the energy radiated back into the atmosphere is
mostly infrared radiation. Certain gases in the atmosphere,
including water vapor and carbon dioxide, allow visible
light to pass through but absorb most infrared radiation.
These gases radiate some of this absorbed energy back to
Earth’s surface, warming the lower atmosphere in a
process called the greenhouse effect. Without the greenhouse
effect, Earth’s surface would be much cooler than it is.
Energy is transferred within the troposphere in three ways:
radiation, convection, and conduction. As the How It Works box on
page 756 explains, these processes work together to heat the troposphere.
Section Resources
Print
• Reading and Study Workbook With
Math Support, Section 24.3
• Transparencies, Section 24.3
Day or
Night?
a. ?
c. ?
Direction of
Air Movement
b. ?
d. ?
20% of incoming
sunlight absorbed
by clouds and gases
50%
absorbed
by surface
5%
reflected
by surface
25%
reflected by
clouds, dust,
and gases
Most energy
absorbed by the
surface is reradiated
back into the
atmosphere.
Figure 11 About half of the
sunlight that reaches Earth is
absorbed by the surface. The rest
is either reflected back into space
or absorbed in the atmosphere.
Weather and Climate 755
Technology
• Interactive Textbook, Section 24.3
• Presentation Pro CD-ROM, Section 24.3
• Go Online, NSTA SciLinks, Winds
Section 24.3
1
FOCUS
Objectives
24.3.1 Describe the processes by
which solar energy heats the
troposphere.
24.3.2 Identify local and global winds
and explain how they are
produced.
Build Vocabulary
Concept Map Have students construct
a concept map connecting the main
topic of Wind to local winds and global
winds. Have students expand the map as
they read to include sea breeze, land
breeze, monsoon, and jet stream.
Reading Strategy
a. Day b. Cool air moves toward land.
c. Night d. Cool air moves toward
the water.
2
Reading Focus
INSTRUCT
Energy in the
Atmosphere
Build Reading Literacy
L2
L2
L1
Summarize Refer to page 598D in
Chapter 20, which provides the
guidelines for summarizing.
Tell students that pp. 755–756 cover
technical material, and that summarizing
it will help them pull together the main
ideas and avoid getting lost in the
details. Allow students to choose
whether they would like to present a
verbal, written, or pictorial summary.
Point out to students that the bolded
key concepts on p. 755 and A, B, and C
in How It Works on p. 756 contain
important information that should be
included in a summary. Intrapersonal
L2
Students may think that the greenhouse
effect is always bad, possibly because it is
often mentioned in conjunction with
global warming. Have students write a
paragraph describing what Earth would
be like without the greenhouse effect.
(Earth’s surface would be much colder,
possibly inhospitable to life.) Logical
Weather and Climate 755

Section 24.3 (continued)
Energy Transfer in
the Troposphere L2
Solar radiation heats Earth’s surface,
which, in turn, transfers energy to the
troposphere. Energy is released by Earth’s
surface in the form of infrared radiation.
Atmospheric gases—such as water vapor,
carbon dioxide, and methane—absorb
infrared radiation. These greenhouse
gases also reradiate some of that energy
back to Earth’s surface. This transfer of
energy by radiation is very efficient, and
is the primary mechanism by which
the troposphere is heated. Because in
convection some heat energy is converted
into kinetic energy, convection
is a less efficient way of transferring heat
to the troposphere. Because air is not a
good heat conductor, conduction is the
least significant way in which heat is
transferred in the troposphere.
Interpreting Diagrams The sun
Visual
For Enrichment
Students can research the specific
wavelengths of infrared radiation that
are absorbed by each of the major gases
in the atmosphere. Verbal, Portfolio
Integrate Physics
756 Chapter 24
L3
L2
The sun emits energy over a wide range
of wavelengths, from radio waves
through X-rays. Slightly more than half
of the sun’s radiation is in the visible and
ultraviolet spectra (10–750 nm). Most
of the radiation in this range passes
through the atmosphere and is absorbed
by Earth’s surface, heating the land and
water. These surfaces then radiate energy
back into the atmosphere in the infrared
spectrum (750–1000 nm). Certain gases
in the atmosphere, including carbon
dioxide and water vapor, efficiently
absorb infrared radiation. This process is
called the greenhouse effect, which is
the accumulation of heat in the lower
atmosphere through the radiation and
reradiation of energy. Ask, Why is
only 20% of incoming solar energy
absorbed by the atmosphere? (Most
solar radiation is in the visible and ultraviolet
range, which most atmospheric
gases cannot absorb.) Which has more
energy, infrared or ultraviolet
radiation? (Ultraviolet) Logical
Energy Transfer in
the Troposphere
Energy is transferred within the troposphere by radiation,
conduction, and convection. Radiation from the sun heats
Earth’s surface, which then radiates heat skyward. The air
in direct contact with Earth’s surface is heated by
conduction. Warm air near the surface expands and rises
and cooler, denser air sinks, forming convection currents
that move heat through the troposphere.
Interpreting Diagrams Where does most of the energy
in the troposphere originally come from?
Radiation Much of the sun’s
radiation reaches Earth’s
surface, where it heats the land and
water. Land and water radiate heat
back into the atmosphere.
756 Chapter 24
Conduction The conduction
process transfers heat from land
and water directly to the few meters
of air nearest Earth’s surface.
Facts and Figures
Radiation and Temperature The reason
that the sun and the Earth emit different forms
of radiation is because they have different
temperatures. Hotter objects emit shorterwavelength
radiation. Colder objects emit
longer-wavelength radiation. The sun’s hot
surface emits radiation in the shorter-
Solar
radiation
Infrared
radiation
Greenhouse effect
When Earth’s surface is heated,
much of this energy is radiated
back as infrared radiation. Some
of this radiation is absorbed by
gases in the atmosphere. The
process by which gases hold
heat in the air is called the
greenhouse effect.
Convection Convection moves heat
through the troposphere. As surface
air is heated by radiation and conduction,
rising warm air is replaced by denser,
downward-flowing cool air.
wavelength visible and ultraviolet spectrum.
Earth’s cooler surface (including the people on
it) emits radiation in the longer-wavelength
infrared spectrum. Objects that are colder than
freezing emit even longer-wavelength radiation,
in the microwave spectrum.

Wind
What happens when you open a vacuum-packed can of chips? You
hear a rush of air, which is the sound of air moving from the highpressure
area outside the can to the low-pressure area inside the can.
A similar process occurs in the atmosphere.
Air naturally flows from areas of higher pressure to areas of lower
pressure. This flow is wind, which is the mainly horizontal movement
of air. Winds are caused by differences in air pressure. Larger
pressure differences produce stronger winds.
Differences in air pressure are often caused by the unequal heating
of Earth’s surface. As you’ve learned, the atmosphere is warmed largely
by reradiation from Earth’s surface below it. As air is heated, it expands.
As it becomes less dense, air rises. Cooler, denser air flows in to replace
it. This process occurs on both local and global scales, producing local
and global winds.
Local Winds
On a hot summer day, there is often a cool breeze
blowing in from the water to the beach. This breeze
is an example of a local wind, a wind that blows
over a short distance. Local winds are caused by
the unequal heating of Earth’s surface within a
small region.
The breezes that occur where land meets a
large body of water are examples of local winds.
Water has a higher specific heat than land, and therefore
takes longer to heat up and cool down. During
the day, the sun heats the land more quickly than it
heats the water. The air above the land becomes
warmer than the air above the water. The warm air
expands and rises, creating a lower-pressure area
above the land. The cooler air over the water flows
toward the land, creating a sea breeze.
At night, these temperature and pressure conditions
are reversed, as Figure 12 shows. Land cools
off more quickly than water. The cooler air over
land has a higher density than the warmer air over
water. The result is a land breeze, where cooler air
over land moves toward water. Winds are named
for the direction from which they originate—sea
breezes begin over the ocean and land breezes
begin over land.
In which direction does a
land breeze blow?
A
B
B
Sea Breeze
Cooler air
moving toward
the land
Land Breeze
Warm air
rising
Customize for English Language Learners
Flowchart
Tailor your presentation to ELL students by
using a flowchart to help students visualize
the sequence of events with minimal use of
language. Title the flowchart The Path of
For: Links on winds
Visit: www.SciLinks.org
Web Code: ccn-3243
Warm air
rising
Cooler air
Cooler air
moving toward
the water
Figure 12 Sea breezes and land breezes are local
winds. A During the day, air pressure differences due to
unequal heating cause a sea breeze. B At night, pressure
differences are reversed, causing a land breeze.
Comparing and Contrasting How are land
breezes and sea breezes different?
Weather and Climate 757
Energy. Use three boxes entitled Sun, Earth’s
Surface, and Troposphere. Have students fill
in the boxes to explain the energy transfer in
their own words.
Wind
Wind Creation
L2
Purpose Students observe how winds
are produced.
Materials clear plastic tank, plastic
wrap, 2 1-L beakers, water, hot plate
or Bunsen burner, ice, wood splint
(smoker), matches
Procedure Fill one beaker about
halfway with water and heat it to nearly
boiling. Fill the other beaker with ice.
Place the beakers inside the tank on
opposite sides and cover the tank with
plastic wrap. Cut a small hole in the wrap
over the ice large enough to fit the splint
through easily. Ask students which way
they think the wind will blow. Light the
splint, blow on it until it is smoking, and
insert it deep into the tank over the ice.
Expected Outcome The smoke will
sink over the ice, move across the tank,
and rise over the hot water, making the
movement of air in the convection
current visible. Visual, Logical
Local Winds
Build Science Skills
L2
Designing Experiments Have groups
of students design an experiment to test
whether a breeze over a lake is produced
by the same process described on p. 757.
Provide them with the following headings
to fill in: Hypothesis, Materials, Procedure,
and Observations. Students’ experiments
may include observations of air pressure,
temperature, and/or wind direction.
Interpersonal, Portfolio
Download a worksheet on winds
for students to complete, and find
additional teacher support from
NSTA SciLinks.
Answer to . . .
Figure 12 They occur at different
times and move in opposite directions.
the water.
A land breeze blows
from the land toward
Weather and Climate 757

Section 24.3 (continued)
Global Winds
Use Visuals
Figure 13 Identify 0, 30 S, 60 S,
30 N, and 60 N. Explain that the global
wind patterns shown in this figure remain
fairly constant throughout the year,
though the change of seasons does have
some effect on the circulation of the
atmosphere. Heating patterns change as
the seasons change, and patterns of air
pressure (and thus wind) change as a
result. Point out that convection cells are
vertical and reach to the top of the
troposphere. Point to the convection cell
between 0 and 30 S and ask, Which
direction is this wind pattern felt on
the ground? (Northwest) In which
regions of Earth are the global winds
moving generally from the east in a
westerly direction? (0°–30° N, 0°–30° S,
60° N–90° N, and 60° S–90° S) Why are
the arrows in the 0–30 S region (over
South America) curving to the left? (The
northerly winds curve westward as the result
of Earth’s rotation—or the Coriolis effect.)
This figure can be used to help explain
the movements of fronts and storms in
Section 24.5 and global climate patterns
in Section 24.7. Visual
Convection Cells
758 Chapter 24
L1
L2
Purpose Students observe a convection
cell produced in water.
Materials 1-L beaker, water, hot plate
or Bunsen burner, 5–10 drops of food
coloring
Procedure Fill the beaker with
approximately 750 mL water. Place the
beaker on a hot plate or Bunsen burner
and heat water until some steam is rising,
but before boiling. Turn off the heat
source. Explain that convection cells are
common phenomena in both air and
water on many scales, from global to
local winds, and from ocean currents to
coffee cups. Slowly add the drops of food
coloring (they will spread quickly, so only
add enough to make the convection cell
visible). Ask students to explain what is
causing the convection cell.
Expected Outcome The food coloring
will reveal the circulation of water in a
convection cell before dissipating. This
cell is produced when warm water
touches surface air, cools, and sinks again
to the bottom. Visual, Logical
Warm air rises
at the equator until
it reaches the top
of the troposphere.
The circulating
air patterns
are called
“convection cells.”
Figure 13 Earth is surrounded by
a set of global wind belts.
Figure 14 For hundreds of years,
sailing ships have relied on global
winds to transport cargo across
the oceans.
Interpreting Visuals Which
band of global winds would a
sailing ship use to move cargo
from Canada to Europe?
758
Facts and Figures
Dry air sinks
over the world’s
deserts.
Where the Wind Dies While trade winds
and westerlies occur where convection cells
blow across the surface, there are also areas
where the wind dies out. These occur where
the convection cells produce areas of rising or
sinking air, such as at the equator, at 30 north
and south latitude, and at 60 north and south
latitude. The area of low winds at the equator
is referred to as the doldrums, and the low
Trade winds
Westerlies
Westerlies
Earth’s rotation
Doldrums
Trade winds
Polar easterlies
Very cold air sinks
at the poles and flows
outward, creating winds
called polar easterlies.
The area where
the trade winds
die out is known
as the doldrums.
Global Winds
Winds that blow over long distances from a specific direction are
called global winds. These winds are part of a worldwide pattern of
air circulation. Global winds are caused by the unequal heating of
Earth’s surface across a large region.
Convection Cells Global winds move in a series of huge bands
called convection cells. As you can see in Figure 13, these bands look
like loops from the side. These bands are caused by temperature variations
across Earth’s surface. At the equator, for example, temperatures
tend to be warmer than at other latitudes. Warm air rises at the equator,
creating a low-pressure region. This warm air is replaced by cooler
air brought by global winds blowing near the surface. Higher in the
atmosphere, air blows away from the equator toward the poles. Similar
convection cells cover large bands of latitude across Earth.
The trade winds are wind belts just north and south of the equator.
In the Northern Hemisphere, they blow from the northeast to the southwest.
The prevailing westerlies occur between 30° and 60° latitude in
both hemispheres. These winds generally blow from west to east over
much of North America. The polar easterlies extend from 60° latitude to
the poles in both hemispheres. Trade winds, westerlies, and polar
easterlies are examples of global winds.
wind regions at 30 north and south latitude
are called the horse latitudes. This name has
a gruesome historical origin. Colonial sailors
traversing the Atlantic would frequently get
stuck around 30 N when the wind died. To
survive at sea with a limited supply of fresh
water on board (and to lighten the ship’s
weight) the sailors would throw a few horses

If Earth were not rotating on its axis, global winds would move in
roughly straight paths from the poles to the equator. However, because
Earth rotates, global winds move in a curved path between the poles
and the equator. Notice in Figure 13 that global winds curve to the
right in the Northern Hemisphere and to the left in the Southern
Hemisphere. The curving effect that Earth’s rotation has on all freemoving
objects, including global winds, is called the Coriolis effect. If
Earth were not rotating, a rocket launched from the North Pole toward
the equator would move in a straight line, as shown in Figure 15.
However, because Earth is rotating underneath the rocket, the rocket
would appear to an observer on Earth to curve to the right. Similary,
Earth’s rotation causes global winds to curve.
Monsoons Seasonal changes in the heating of Earth’s surface affect
the circulation of the atmosphere. A monsoon is a wind system that is
characterized by seasonal reversal of direction. Monsoons are similar to
land and sea breezes except that they occur on a much wider scale and
longer time frame. For example, the summer monsoon that occurs over
much of South and Southeast Asia blows warm, humid air from the
ocean onto land. As this air rises over land, it cools and brings heavy
rainfall to parts of that region. In winter the monsoon reverses, blowing
from land onto water and bringing drier weather.
Jet Stream Global wind patterns are also affected by fast-moving
streams of air at high altitudes. A belt of high-speed wind in the upper
troposphere is called a jet stream. Jet streams are caused by great differences
in air pressure that develop at high altitudes.
Section 24.3 Assessment
Reviewing Concepts
1. What happens to solar energy when it
reaches Earth’s atmosphere?
2. Explain how energy is transferred within
the troposphere.
3. What causes the wind to blow?
4. How are local winds and global winds
similar? How are they different?
5. What are monsoons, and what causes them?
Critical Thinking
6. Relating Cause and Effects How does the
Coriolis effect influence global wind patterns?
Section 24.3 Assessment
1. Solar energy is distributed in three ways:
some is reflected, some is absorbed by the
atmosphere, and some is absorbed by Earth’s
surface.
2. Energy is transferred within the troposphere
by radiation, convection, and conduction.
3. Wind is caused by differences in air
pressure between different locations.
4. Both local and global winds are produced
by differences in air pressure that result from
A
Non-rotating Earth
Movement of
rocket
Movement of
rocket
Equator
Equator
7. Applying Concepts You and your family
vacation at a cabin on the shore of a large
lake. At night, you notice that a breeze blows
over your cabin toward the lake. Explain what
causes the wind to blow in that direction.
Thermal Energy Recall what you learned
in Chapter 16 about the specific heat of
water. Use this information to explain why
land and sea breezes undergo daily reversals
in direction.
B
Direction of
Earth’s rotation
Rotating Earth
Figure 15 The Coriolis effect
causes free-moving objects, such
as rockets and global winds, to
move in a curved path. A A rocket
launched from the North Pole
toward the equator would move
in a straight line if Earth were not
rotating. B The Coriolis effect
causes such a rocket to appear to
curve to the right. Similarly, the
Coriolis effect causes global winds
to curve to the right in the
Northern Hemisphere.
Weather and Climate 759
the unequal heating of Earth’s surface. For
local winds, this unequal heating takes place
within a small region, and the resulting wind
blows over only a short distance. In contrast,
global winds are caused by unequal heating
across a large area, and blow over long
distances for extended periods of time.
5. Monsoons are large wind systems that
have seasonal reversals of direction. They are
caused by air pressure differences that result
from the unequal heating of air above the
ocean and land.
3
ASSESS
Evaluate
Understanding
Ask students to create a flowchart
describing how local winds are created.
Reteach
L2
L1
Use Figure 13 to explain convection cells
and the Coriolis effect.
Water has a high specific heat, higher
than that of land. As a result, water
warms up slower during the day and
cools down slower at night than land.
This affects the temperature of the air
above the water and land. Land and sea
breezes are created by this daily cycle of
unequal heating of air in coastal regions.
If your class subscribes to
the Interactive Textbook, use it to
review key concepts in Section 24.3.
Answer to . . .
Figure 14 A sailing ship would use
the westerlies to move cargo from
Canada to Europe.
6. The Coriolis effect causes global winds to
turn to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
7. When night falls, the lake water remains
warm as the land cools off quickly. The warmer
air over the water has a lower pressure than
the cooler air over the land. The result is a land
breeze, which is when the cooler air over land
moves toward water.
Weather and Climate 759