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Table of Contents
Chapter: The Atmosphere
in Motion
Section 1: The Atmosphere
Section 2: Earth’s Weather
Section 3: Air Masses and Fronts
The Atmosphere
1
Investigating Air
• The atmosphere—the layer of gases
surrounding Earth—provides Earth with all
the gases necessary to support life.
The Atmosphere
1
Investigating Air
• Galileo Galilei (1564-1642), an Italian
astronomer and physicist, suspected that air
was more than just
empty space.
• He weighed a flask,
then injected air into
it and weighed it
again.
The Atmosphere
1
Investigating Air
• Galileo observed that the flask weighed more
after injecting the air.
• He concluded that
air must have weight
and therefore must
contain matter.
The Atmosphere
1
Investigating Air
• Today scientists know that air stores and
releases heat and holds moisture.
• Because it has weight, air can exert pressure.
The Atmosphere
1
Composition of the Atmosphere
• Because it is composed of matter and has
mass, the atmosphere is subject to the pull
of gravity.
• The atmosphere is composed of a mixture of
gases, liquid water, and microscopic particles
of solids and other liquids.
The Atmosphere
1
Gases
• This graph shows the gases found in the
atmosphere.
• Nitrogen (N2) is
the most abundant
gas—it makes up
about 78 percent
of the atmosphere.
The Atmosphere
1
Gases
• Oxygen (O2), the gas necessary for human
life, makes up about 21 percent.
• Of the trace gases, two have important roles
within the
atmosphere.
• Water vapor (H2O)
makes up from 0.0
to 4.0 percent of the
atmosphere and is
critical to weather.
The Atmosphere
1
Gases
• Carbon dioxide is needed for plants to
make food.
• Carbon dioxide in the atmosphere absorbs
heat and emits it back toward Earth’s
surface, helping keep Earth warm.
The Atmosphere
1
Aerosols
• Solids such as dust, salt, and pollen and
tiny liquid droplets such as acids in the
atmosphere are called aerosols (AR uh
sahlz).
• Salt enters the atmosphere when wind blows
across the oceans.
The Atmosphere
1
Aerosols
• Pollen enters the
atmosphere when it is
released by plants.
• Volcanoes add many
aerosols to the
atmosphere.
• Human activities also
release aerosols into
the air.
The Atmosphere
1
Layers of the Atmosphere
• The atmosphere is divided into layers
based on temperature changes that occur
with altitude.
• The white
temperature scale
shows temperatures
in the thermosphere
and exosphere.
The Atmosphere
1
Troposphere
• The troposphere (TROH puh sfihr) is the
atmospheric layer closest to Earth’s
surface.
• It extends upward to about 10 km.
• The troposphere contains about three-fourths
of the matter in Earth’s entire atmosphere
and nearly all of its clouds and weather.
The Atmosphere
1
Troposphere
• About 50 percent of the Sun’s energy
passes through the troposphere and reaches
Earth’s surface.
• Temperatures in the troposphere are usually
warmest near the surface and tend to cool
as altitude increases.
The Atmosphere
1
Stratosphere
• The stratosphere extends from about 10 km
to about 50 km above Earth’s surface.
• Most atmospheric ozone is contained in the
stratosphere.
• Without the ozone in this layer, too much
radiation would reach Earth’s surface,
causing health problems for plants and
animals.
The Atmosphere
1
Upper Layers
• Above the stratosphere is the mesosphere
(ME zuh sfihr).
• This layer extends
from approximately
50 km to 85 km above
Earth’s surface.
The Atmosphere
1
Upper Layers
• The thermosphere (THUR muh sfihr) is
above the mesosphere.
• Temperatures increase rapidly in this layer to
more than 1700°C.
• The thermosphere
layer filters out
harmful X rays and
gamma rays from
the Sun.
The Atmosphere
1
Upper Layers
• A part of the thermosphere and mesosphere
is called the ionosphere (i AH nuh sfihr).
• This layer of ions is
useful because it can
reflect AM radio waves
making long distance
communication possible.
The Atmosphere
1
Upper Layers
• The outermost layer of the atmosphere is
the exosphere.
• It extends outward
to where space
begins and contains
few atoms.
• No clear boundary
separates the
exosphere form
space.
The Atmosphere
1
Earth’s Water
• Earth’s surface is about 70
percent water.
• As the table shows, water
exists as solid snow or ice
in glaciers.
• In oceans, lakes and rivers
water exists as a liquid and
in the atmosphere it exists
as gaseous vapor.
The Atmosphere
1
The Water Cycle
• Earth’s water is in constant motion in a neverending process called the water cycle.
The Atmosphere
1
The Water Cycle
• Water on Earth’s surface—in oceans, lakes,
rivers, and streams—absorbs energy and
stores it as heat.
The Atmosphere
1
The Water Cycle
• When water has enough heat energy, it
changes from liquid water into water vapor
in a process called evaporation.
The Atmosphere
1
The Water Cycle
• Water vapor then enters the atmosphere.
• Evaporation occurs from all bodies of water,
no matter how large or small.
The Atmosphere
1
The Water Cycle
• Water also is transferred into the atmosphere
from plant leaves in a process called
transpiration.
The Atmosphere
1
The Water Cycle
• Eventually, the water molecules change back
into droplets of liquid water. This process is
called condensation.
The Atmosphere
1
The Water Cycle
• Eventually, these droplets become large
enough to be visible, forming a cloud.
• If the water droplets continue to grow,
they become too large to remain
suspended in the atmosphere and fall to
Earth as precipitation.
Section Check
1
Question 1
Identify the layers of the atmosphere and
briefly explain their characteristics.
Section Check
1
Answer
Troposphere: weather occurs in this layer;
Stratosphere: contains atmospheric ozone,
absorbs ultraviolet radiation from the Sun;
Mesosphere: contains little ozone and therefore
absorbs very little radiation; Thermosphere:
filters out harmful X-rays and gamma rays from
the Sun; Ionosphere: reflects AM radio waves
making long-distance communication possible;
Exosphere: extends into space
Section Check
1
Question 2
How does an erupting volcano affect Earth’s
atmosphere?
Section Check
1
Answer
When a volcano erupts, it releases tiny
particles of ash or aerosols into the atmosphere.
This ash creates a blanket over Earth’s surface
that reflects solar energy and influences
weather and climate.
Section Check
1
Question 3
What is the process called when water changes
from a liquid into a vapor?
A. condensation
B. evaporation
C. precipitation
D. transpiration
Section Check
1
Answer
The answer is B. Evaporation occurs when
water has enough heat energy to change from a
liquid into water vapor
Earth’s Weather
2
Weather
• Weather describes the current condition
of the atmosphere.
• Factors of weather include temperature,
cloud cover, wind speed, wind direction,
humidity, and air pressure.
Earth’s Weather
2
Temperature
• Temperature is a measure of the average
movement of molecules.
• The faster
they’re moving,
the higher the
temperature is.
Earth’s Weather
2
Energy Transfer
• The transfer of energy that results when
molecules collide is called conduction.
• It is conduction that transfers heat from
Earth’s surface to those molecules in the air
that are in contact with it.
Earth’s Weather
2
Energy Transfer
• Rising air cools as it gets higher.
• If it becomes cooler than the surrounding
air, it will sink.
• The process of warm air rising and cool air
sinking is called convection.
• It is the main way heat is transferred
throughout the atmosphere.
Earth’s Weather
2
Atmospheric Pressure
• Air pressure decreases with altitude in the
atmosphere.
• This is because
as you go
higher, the
weight of the
atmosphere
above you is
less.
Earth’s Weather
2
Atmospheric Pressure
• Temperature and pressure are related.
• When air is heated, its molecules move
faster, and the air expands. This makes the
air less dense, which is why heated air gets
moved upward.
Earth’s Weather
2
Atmospheric Pressure
• Less dense air also exerts less pressure on
anything below it, creating lower pressure.
• Therefore, rising air generally means lower
pressure and sinking air means higher
pressure.
Earth’s Weather
2
Humidity
• The amount of water vapor in the atmosphere
is called humidity.
• The graph shows
how temperature
affects how much
moisture can be
present in the air.
Earth’s Weather
2
Humidity
• When air is holding as much water vapor as
it can, it is said to be saturated and
condensation can occur.
• The temperature at which this takes place is
called the dew point.
Earth’s Weather
2
Relative Humidity
• Relative humidity is a measure of the
amount of water vapor that is present
compared to the amount that could be held
at a specific temperature.
• When air is holding all of the water vapor
it can at a particular temperature, it has
100 percent relative humidity.
Earth’s Weather
2
Clouds
• Clouds form when air rises, cools to its dew
point, and becomes saturated.
• If the temperature is not too cool, the clouds
will be made of small drops of water.
• If the temperature is cold enough, clouds can
consist of small ice crystals.
Earth’s Weather
2
Low Clouds
• The low-cloud group consists
of clouds that form about
2,000 m or less in altitude.
• These clouds include the
cumulus (KYEW myuh
lus), layered stratus (STRA
tus), and nimbostratus
(nihm boh STRA tus) cloud.
Earth’s Weather
2
Low Clouds
• Fog is a type of stratus cloud
that is in contact with the
ground.
Earth’s Weather
2
Middle Clouds
• Clouds that form between
about 2, 000 m and 8,000 m
are known as the middle-cloud
group.
• Their names often have the
prefix alto- in front of them,
such as altocumulus and
altostratus.
Earth’s Weather
2
Middle Clouds
• Sometimes they contain
enough moisture to produce
light precipitation.
Earth’s Weather
2
High and Vertical Clouds
• Some clouds occur in air that
is so cold they are made up
entirely of ice crystals.
• These are known as the highcloud group.
• They include cirrus (SIHR
us) clouds and cumulonimbus
(kyew myuh loh MIHM bus).
Earth’s Weather
2
High and Vertical Clouds
• Cumulonimbus clouds create
the heaviest precipitation of
all.
Earth’s Weather
2
Precipitation
• When drops of water or crystals of ice
become too large to be suspended in a cloud,
they fall as precipitation.
• The type of precipitation that falls depends
on the temperature of the atmosphere.
• For example, if air aloft is above freezing
while air near Earth’s is below freezing,
freezing rain might occur.
Earth’s Weather
2
Wind
• Typically, air moves from high-pressure
areas toward low-pressure areas.
• Because pressure and temperature are
directly related, wind can be thought of
simply as air moving form one temperature
or pressure area to another.
Earth’s Weather
2
Wind
• The greater the difference in temperature or
pressure between two areas, the stronger the
winds that blow between them will be.
• Wind speed is measured by and instrument
called an anemometer (an uh MAH muh tur).
• The fastest wind speed ever measure was
371 km/h.
Earth’s Weather
2
Global Air Circulation
• In any given year, the Sun’s rays strike Earth
at a higher angle near the equator than near
the poles.
• Warm air flows
toward the
poles from the
tropics and cold
air flows toward
the equator
form the poles.
Earth’s Weather
2
Global Air Circulation
• Because Earth rotates, this moving air is
deflected to the right in the northern
hemisphere and to the left south of the
equator.
• This is known as the Coriolis (kor ee OH
lus) effect.
Earth’s Weather
2
Surface Winds
• Farther from the equator, at about 30° latitude,
the air is somewhat cooler.
• As this air flows, it is turned by the Coriolis
effect, creating
steady winds
called the trade
winds.
• They blow in a
general east-towest direction.
Earth’s Weather
2
Westerlies and Easterlies
• Major wind cells also are located between
30° and 60° latitude north and south of the
equator.
• They blow
from the west
and are called
the prevailing
westerlies.
Earth’s Weather
2
Westerlies and Easterlies
• Many of Earth’s major weather systems form
along these boundaries, so these regions are
known for frequent storms.
Earth’s Weather
2
Westerlies and Easterlies
• Near the poles, cold, dense air sinks and flows
away from the poles.
• As the cold air flows away from the poles, it
is turned by the
Coriolis effect.
These winds, the
polar easterlies,
blow from the
east.
Earth’s Weather
2
Jet Streams
• Jet streams are like giant rivers of air.
• They blow near the top of the troposphere
from west to east at the northern and southern
boundaries of the prevailing westerlies.
• Their positions in latitude and altitude change
from day to day and from season to season.
Earth’s Weather
2
Other Winds
• Slight differences in pressure create gentle
breezes.
• Great differences create strong winds.
• The strongest winds occur when air ruches
into the center of the low pressure. This
can cause severe weather like tornadoes
and hurricanes.
Section Check
2
Question 1
How are temperature and pressure related?
Section Check
2
Answer
When air is heated, its molecules move faster
and the air expands. This makes the air less
dense and it rises. As this less dense air rises
and exerts less pressure on anything below it.
Cooled air becomes denser and sinks as the
molecules slow down and move closer
together. This cool air exerts more pressure on
anything below it.
Section Check
2
Question 2
Which winds blow in an east-to-west direction
and are formed by cool, sinking air?
Section Check
2
A. polar easterlies
B. prevailing westerlies
C. surface winds
D. trade winds
Section Check
2
Answer
The answer is D. Cooler air sinks toward the
equator and is turned by the Coriolis effect
creating trade winds.
Section Check
2
Question 3
Which are responsible for producing heavy
precipitation and thunderstorms?
A. altostratus clouds
B. cirrus clouds
C. cumulonimbus clouds
D. nimbostratus clouds
Section Check
2
Answer
The answer is C. Cumulonimbus clouds have
high vertical development and produce
thunder, lightning, and heavy rain.
Air Masses and Fronts
3
Air Masses
• Weather changes quickly when a different
air mass enters an area.
• An air mass is a large body of air that
develops over a particular region of Earth’s
surface.
Air Masses and Fronts
3
Types of Air Masses
• A mass of air that
remains over a
region for a few
days acquires the
characteristics of
the area over which
it occurs.
• Six major air masses
affect North America.
Air Masses and Fronts
3
Fronts
• Where air masses of different temperatures
meet, a boundary between them, called a
front, is created.
Click image to view movie.
Air Masses and Fronts
3
Cold Fronts
• When a cold air mass advances and pushes
under a warm air mass, the warm air is
forced to rise.
• The boundary is
known as a cold
front.
• A narrow band of
violent storms can
result from a cold
front.
Air Masses and Fronts
3
Warm Fronts
• If warm air is advancing into a region of
colder air, a warm front is formed.
• As the warm air mass
moves upward, it
cools. Water vapor
condenses and
precipitation occurs
over a wide area.
Air Masses and Fronts
3
Stationary Fronts
• A stationary front is a front where a warm
air mass and a cold air mass meet but neither
advances.
• Cloudiness and
precipitation
occur along the
front.
Air Masses and Fronts
3
Occluded Fronts
• An occluded front forms when a fast-moving
cold front overtakes a slower warm front.
• All types of occluded fronts can produce
cloudy weather with precipitation.
Air Masses and Fronts
3
High- and Low-Pressure Centers
• In areas where pressure is high, air sinks.
• As it spreads, the Coriolis effect turns the
air in a clockwise direction in the northern
hemisphere.
• Because the air is sinking, air near a
high-pressure center is usually dry with
few clouds.
Air Masses and Fronts
3
High- and Low-Pressure Centers
• As air flows into a low-pressure center, it
rises and cools.
• Eventually, the air reaches its dew point and
the water vapor condenses, forming clouds
and precipitation.
• Air circulates in a counterclockwise
direction in the northern hemisphere in a
low-pressure center.
Air Masses and Fronts
3
Severe Weather—Thunderstorms
• Thunderstorms develop from cumulonimbus
clouds.
• Falling droplets
collide with
other droplets
and grow bigger.
• As these larger
droplets fall, they
cool the surrounding air, creating downdrafts
that spread out at the surface.
Air Masses and Fronts
3
Severe Weather—Thunderstorms
• These are the strong winds associated with
thunderstorms.
• Lightning and thunder also are created in
cumulonimbus clouds.
Air Masses and Fronts
3
Tornadoes
• A tornado is a violent, whirling wind that
moves in a narrow path over land.
• The powerful updrafts into the low
pressure in the center of a tornado act like
a giant vacuum cleaner, sucking up
anything in its path.
Air Masses and Fronts
3
Tornadoes
Air Masses and Fronts
3
Hurricanes
• A hurricane is a large storm that begins as an
area of low pressure over tropical oceans.
• The Coriolis
effect causes
winds to rotate
counterclockwise
around the center
of the storm.
Air Masses and Fronts
3
Hurricanes
• As the storm moves, it pulls in moisture.
• The heat energy from the moist air is
converted to wind.
• The sustained winds in a hurricane can
reach 250 km/h with gusts up to 300 km/h.
Air Masses and Fronts
3
Hurricanes
• When a hurricane
passes over land,
high winds,
tornadoes, heavy
rains, and storm
surge pound the
affected region.
Air Masses and Fronts
3
Hurricanes
• After the storm
begins traveling
over land,
however, it no
longer has the
warm, moist air
to provide it
with energy, and
it begins losing
power.
Air Masses and Fronts
3
Weather Safety
• Using technology such as Doppler radar, as
well as weather balloons, satellites, and
computer, the position and strength of storms
are watched constantly.
• If the National Weather Service believes
conditions are right for severe weather to
develop in a particular area, it issues a severe
weather watch.
Air Masses and Fronts
3
Watches and Warnings
• Watches and warnings are issued for severe
thunderstorms, tornadoes, tropical storms,
hurricanes, blizzards, and floods.
• Local radio and television stations announce
watches and warnings, along with NOAA
(NOH ah) Weather Radio.
Air Masses and Fronts
3
Watches and Warnings
• During a watch, stay tuned to a radio or
television station and have a plan of action in
case a warning is issued.
• If the National Weather Service does issue a
warning, take immediate action to protect
yourself.
Section Check
3
Question 1
Where is a front most likely to occur?
Answer
Fronts occur where air masses of different
temperatures meet.
Section Check
3
Question 2
Explain how lightning forms.
Section Check
3
Answer
Lightning is the flow of energy that occurs
between areas of opposite electrical charge.
During a storm, the bottom storm cloud has a
negative charge while the ground has a positive
charge. The negative charge rushes toward the
ground and the positive charge rushes toward
the cloud. These charges meet in the middle
and create lightning.
Section Check
3
Question 3
If conditions are right for severe weather to
develop in a particular area, what will the
National Weather Service issue?
A. an advisory
B. Doppler radar
C. a warning
D. a watch
Section Check
3
Answer
The answer is D. A severe weather watch is
issued if the conditions are right for severe
weather to develop in an area. A severe
weather warning is issued if severe weather is
already occurring.
Chapter 12 Review 1 of 2
• Atmosphere: layer of gases surrounding
Earth.
• Air pressure is due to gravity.
• Most of the ozone of the atmosphere is
located in the stratosphere.
• The majority of water on Earth can be
found in oceans.
• Convection: process of warm air rising and
cool air sinking.
Chapter 12 Review 2 of 2
• Air pressure decreases with altitude in the
atmosphere.
• If the amount of water vapor doesn’t change,
relative humidity increases as air cools.
• Precipitation can be in the form of rain, freezing
rain, sleet, snow, or hail.
• Coriolis effect: moving air is deflected to the right
in the northern hemisphere.
• In the northern hemisphere, air circulates in a
counterclockwise direction.