CHAPTER 11

LESSON 3
Earth’s Atmosphere

Air Currents
Key Concepts
• How does uneven
heating of Earth’s surface What do you think? Read the two statements below and decide
result in air movement? whether you agree or disagree with them. Place an A in the Before
column if you agree with the statement or a D if you disagree. After
• How are air currents on
you’ve read this lesson, reread the statements to see if you have
Earth affected by Earth’s changed your mind.
spin?
Before Statement After
• What are the main wind
belts on Earth? 5. Uneven heating in different parts of the
atmosphere creates air circulation patterns.
6. Warm air sinks and cold air rises.

Finding Main Ideas

Highlight the main idea of
Global Winds
each paragraph in this Wind patterns can be global or local. There are great wind belts
lesson. Reread the that circle Earth. The energy that causes this large movement of air
sentences to review the comes from the Sun.
lesson.
Unequal Heating of Earth’s Surface
The Sun’s energy warms Earth. Not all areas on Earth’s surface
Reading Check receive the same amount of energy from the Sun. The amount of
1. Identify What is the energy an area gets depends on the Sun’s angle. Energy coming
source of energy of global from the rising or setting Sun is not very strong. But Earth warms
wind belts? quickly when the Sun is high in the sky around noon.
Sunlight Areas in low latitudes near the equator are referred to as
the tropics. Sunlight strikes Earth’s surface there at a high angle. It
Copyright © McGraw-Hill Education.

is nearly a 90° angle all year-round. As a result, the tropics receive

more sunlight per unit of surface area than other places on Earth.
This causes the land, the water, and the air at the equator to
always be warm.
At latitudes near the North Pole and the South Pole, sunlight
strikes Earth’s surface at a low angle. Sunlight is spread over a
larger surface area than in the tropics. This means that the poles
receive very little energy per unit of surface area and so they are
cooler.

346 Earth’s Atmosphere Air Currents

 Wind Recall that warm air rises and cold air sinks. Warm air is
less dense than cold air. Rising warm air puts less pressure on
Earth than cooler air. Air pressure is usually low over the tropics
because it is usually warm there. Air pressure is usually high over
colder areas such as the North and South Poles. This difference in
pressure creates wind. Wind is the movement of air from areas of
high pressure to areas of low pressure. Global wind belts
influence both climate and weather on Earth.
Key Concept
Check
Global Wind Belts 2. Explain How does
The figure below shows the three-cell model used to describe uneven heating of Earth’s
circulation in Earth’s atmosphere. Three cells exist in both the surface result in air
movement?
northern hemisphere and the southern hemisphere.
In the first cell, hot air near the equator moves to the top of the
troposphere. Then, the air moves toward the poles until it cools
and moves back to Earth’s surface near 30° latitude. Most of the air
in this convection cell then returns to the equator near Earth’s
surface.
The third cell is at the highest latitudes and is also a convection
cell. Air from the poles moves toward the equator along Earth’s
surface. Warmer air is pushed upward by cooler air near 60°
latitude. Make a shutterfold book to
describe Earth’s global
The second cell is between 30° and 60° latitude and is not a
wind belts and to explain
convection cell. Its motion is driven by the other two cells. Imagine how they circulate.
rolling cookie dough between your hands. Your hand represent the
first and third cells. The cookie dough is the second cell. This
second cell moves in much the same way as the dough.
Copyright © McGraw-Hill Education.

Visual Check
3. Identify Which wind
belt do you live in?

Air Currents Earth’s Atmosphere 347

 The Coriolis Effect
What would happen if you threw a ball to someone sitting across
from you on a moving merry-go-round? When the ball reached the
Key Concept opposite side, the person would have moved. The ball would have
Check appeared to have curved. Like a merry-go-round, the rotation of
4. Describe How are air Earth causes moving air and water to appear to move to the right in
currents on Earth affected the northern hemisphere and to the left in the southern hemisphere.
by Earth’s spin? This is called the Coriolis effect. The difference between high and
low pressure and the Coriolis effect create distinct wind patterns.
These wind patterns are called prevailing winds.

Prevailing Winds
The three global wind cells in each hemisphere create northerly
and southerly winds. When the Coriolis effect acts on the winds,
the winds blow to the east or the west. These winds are relatively
steady and predictable.
Reading Check
The trade winds are steady winds that flow from east to west
5. Explain What might between 30°N latitude and 30°S latitude. At about 30°N and 30°S
happen to a sailboat caught
latitude, air cools and sinks. This creates areas of high pressure
in the doldrums?
and light, calm winds called the doldrums. Sailboats without
engines can be stranded in the doldrums.
The prevailing westerlies are steady winds that flow from
west to east between latitudes 30°N and 60°N, and 30°S and
60°S. The polar easterlies are cold winds that blow from east to
west near the North Pole and the South Pole.
Key Concept
Check Jet Streams
6. Summarize What are Near the top of the troposphere is a narrow band of high
the main wind belts on winds called the jet stream. Jet streams flow around Earth from
Earth? west to east, often making large loops to the north or the south. Jet
streams also influence weather. They move cold air from the poles
toward the tropics. Jet streams also move warm air from the
tropics toward the poles. Jet streams can move as fast as 300 km/h.

Local Winds
Recall that global winds occur because of pressure differences
Copyright © McGraw-Hill Education.

around the globe. In the same way, local winds occur whenever air
pressure is different from one location to another.

348 Earth’s Atmosphere Air Currents

 Sea and Land Breezes
If you have ever been to a lake or an ocean, you have probably
experienced the connection between temperature, air pressure, and
wind. A sea breeze is wind that blows from the sea to the land
due to local temperature and pressure differences.
The left side of the figure below shows how a sea breeze forms.
Land warms faster than water does. On sunny days, the air over
the land warms by conduction and rises, creating an area of low
pressure. The air over the water sinks. This creates an area of high
pressure because it is cooler. The differences in pressure over the Reading Check
warm land and the cooler water results in a cool wind that blows 7. Compare and
from the sea onto the land. Contrast Explain how
sea and land breezes are
A land breeze is a wind that blows from the land to the sea the same and different.
due to local temperature and pressure differences. The right side
of the figure below shows how a land breeze forms. At night,
the land cools more quickly than the water. The air above the land
also cools more quickly than the air above the water. As a result,
an area of lower pressure forms over the warmer water. In a land
breeze, cool air over land moves toward lower pressure over the
water.

Visual Check
8. Sequence the steps
involved in the formation of
a land breeze.
Copyright © McGraw-Hill Education.

Air Currents Earth’s Atmosphere 349

Mini Glossary
jet stream: a narrow band of high winds located trade winds: steady winds that flow from east to
near the top of the troposphere west between 30°N latitude and 30°S latitude

land breeze: wind that blows from the land to the westerlies: steady winds that flow from west to east
sea due to local temperature and pressure between latitudes 30°N and 60°N, and 30°S and
differences 60°S

polar easterlies: cold winds that blow from the east wind: the movement of air from areas of high
to the west near the North and South Poles pressure to areas of low pressure

sea breeze: wind that blows from the sea to the

land due to local temperature and pressure
differences

1. Review the terms and their definitions in the Mini Glossary. Choose two types of wind and
explain how their names help you remember what they are.

2. Complete the table.

Winds How They Blow and Where They Are

Trade winds

Prevailing westerlies

Polar easterlies

Jet streams
Copyright © McGraw-Hill Education.

Sea breezes

Land breezes

What do you think

Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you Log on to ConnectED.mcgraw-hill.com
agree with the statement or a D if you dis- and access your textbook to find this END OF
agree. Did you change your mind? lesson’s resources. LESSON

350 Earth’s Atmosphere Air Currents

 Lesson 3 Air Currents
Skim Lesson 3 in your book. Read the headings and look at the photos and illustrations. Identify
three things you want to learn more about as you read the lesson. Write your ideas in your Science
Journal.

Global Winds Explain the formation of Earth’s global winds.

I found this on page
. The Sun heats Earth’s surface unevenly because of the

. This uneven

heating causes differences in .

pressure develops over the tropics.

pressure develops over the poles. The movement of air from areas

of high pressure to areas of low pressure is called .

Global wind belts influence

a. b.

Global Winds Belts Assess information about circulation in Earth’s atmosphere.

I found this on page Read each statement below. If the statement is true, write true on the
. line. If the statement is false, write false on the line and rewrite the
underlined portion so that it is true.
Two of the three cells that scientists use to describe circulation of
Earth’s atmosphere are conduction cells.

The first belt begins with warm air rising at the equator and dropping
back to Earth near 30˚ latitude.
Copyright © McGraw-Hill Education.

The third cell, at the lowest latitude, is also a convection cell.

I found this on page Explain the Coriolis effect.

.
Coriolis effect:

Earth’s Atmosphere 351

 Lesson 3 | Air Currents (continued)

Analyze prevailing winds.

Winds Description
I found this on page Trade
.

I found this on page Doldrums

.

I found this on page Westerlies

.

I found this on page Polar

. easterlies

I found this on page Jet stream

.

Local Winds Compare and contrast a sea breeze and a land breeze.
I found this on page
.
Sea Breeze Land Breeze

Both
winds caused
by local
temperature
and pressure
differences Copyright © McGraw-Hill Education.

An airplane pilot flying from California to New York would like to make the
flight in the shortest amount of time possible. What could the pilot do to decrease his travel
time?

352 Earth’s Atmosphere