Download SECTION 2 Air Masses and Fronts

Name
CHAPTER 16
Class
Date
Understanding Weather
SECTION
2 Air Masses and Fronts
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
ES 1j
• How is an air mass different from a front?
• How do fronts affect weather?
What Are Air Masses?
Have you ever been caught outside when it suddenly
started to rain? What causes such an abrupt change in the
weather? Changes in weather are caused by the movement of bodies of air called air masses. An air mass is
a very large volume of air that has a certain temperature
and moisture content.
There are many types of air masses. Scientists classify
air masses by the water content and temperature of the air.
These features depend on where the air mass forms. The
area over which an air mass forms is called a source region.
One source region is the Gulf of Mexico. Air masses that
form over this source region are wet and warm.
Each type of air mass forms over a certain source
region. On maps, meteorologists use two-letter symbols
to represent different air masses. The first letter indicates
the water content of the air mass. The second letter indicates its temperature. The figure below shows the main
air masses that affect North America.
M0
C0
READING CHECK
1. Identify How do
scientists classify air masses?
M0
TAKE A LOOK
C4
M4
STUDY TIP
Summarize As you read,
make a chart comparing the
four kinds of fronts. In your
chart, describe how each
kind of front forms and what
kind of weather it can cause.
M4
MARITIMEMFORMS
OVERWATERWET
POLAR0FORMSOVERTHE
POLARREGIONSCOLD
CONTINENTALCFORMS
OVERLANDDRY
TROPICAL4FORMSOVER
THE4ROPICSWARM
2. Apply Concepts Describe
the temperature and moisture
content of a cT air mass.
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Air Masses and Fronts continued
COLD AIR MASSES
READING CHECK
3. Identify What is the
source region for cP air
masses?
Most of the cold winter weather in the United States
comes from three polar air masses. Continental polar
(cP) air masses form over northern Canada. They bring
extremely cold winter weather. In the summer, cP air
masses can bring cool, dry weather.
Maritime polar (mP) air masses form over the North
Pacific Ocean. They are cool and very wet. They bring
rain and snow to the Pacific Coast in winter. They bring
fog in the summer.
Maritime polar air masses also form over the North
Atlantic Ocean. They bring cool, cloudy weather and
precipitation to New England.
WARM AIR MASSES
Critical Thinking
4. Infer Why don’t warm air
masses form over the North
Atlantic or Pacific oceans?
Four warm air masses influence the weather in the
United States. Maritime tropical (mT) air masses form
over warm areas in the Pacific Ocean, the Gulf of Mexico,
and the Atlantic Ocean. They move across the East Coast
and into the Midwest. In summer they bring heat, humidity, hurricanes, and thunderstorms to these areas.
Continental tropical air masses (cT) form over deserts
and move northward. They bring clear, dry, hot weather
in the summer.
Air mass
How it affects weather
cP from northern Canada
mP from the North Pacific Ocean
mT from the Gulf of Mexico
TAKE A LOOK
5. Identify Fill in the blank
spaces in the table.
cT from the deserts
What Are Fronts?
READING CHECK
6. Define What is a front?
The place where two or more air masses meet is called
a front. When air masses meet, the less dense air mass
rises over the denser air mass. Warm air is less dense
than cold air. Therefore, a warm air mass will generally
rise above a cold air mass. There are four main kinds
of fronts: cold fronts, warm fronts, occluded fronts, and
stationary fronts.
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Air Masses and Fronts continued
COLD FRONTS
A cold front forms when a cold air mass moves under a
warm air mass. The cold air pushes the warm air mass
up. The cold air mass replaces the warm air mass. Cold
fronts can move quickly and bring heavy precipitation.
When a cold front has passed, the weather is usually
cooler. This is because a cold, dry air mass moves in
behind the cold front.
#OLDFRONT
7ARMAIRMASS
#OLDAIRMASS
TAKE A LOOK
7. Describe What happens
to the warm air mass at a
cold front?
A cold front forms
when a cold air mass
pushes a warmer air
mass away. The front
moves in the direction that the cold air
mass is moving.
-OVEMENTOFFRONT
WARM FRONTS
A warm front forms when a warm air mass moves in
over a cold air mass that is leaving an area. The warm air
replaces the cold air as the cold air moves away. Warm
fronts can bring light rain. They are followed by clear,
warm weather.
Warm front
READING CHECK
8. Define What is a warm
front?
Warm air mass
Cold air mass
Movement of front
A warm front forms
when a warm air mass
moves in and replaces
a cold air mass. The
front moves in the
direction the warm air
mass is moving.
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Air Masses and Fronts continued
OCCLUDED FRONTS
An occluded front forms when a warm air mass is caught
between two cold air masses. Occluded fronts bring cool
temperatures and large amounts of rain and snow.
Warm air mass
Warm air mass
Occluded
front
Occluded
front
TAKE A LOOK
9. Describe What happens
to the warm air mass in an
occluded front?
Cold air mass
An occluded front
forms when a warm
air mass is trapped
between two cold air
masses. The cold air
masses move together
and push the warm air
out of the way.
Cold air mass
Movement of front
STATIONARY FRONT
A stationary front forms when a cold air mass and
a warm air mass move toward each other. Neither air
mass has enough energy to push the other out of the way.
Therefore, the two air masses remain in the same place.
Stationary fronts cause many days of cloudy, wet weather.
TAKE A LOOK
10. Infer What do you think
is the reason that stationary
fronts bring many days of the
same weather?
3TATIONARY
FRONT
#OLDAIRMASS
7ARMAIRMASS
A stationary front forms
when air masses stay in
one place.
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Air Masses and Fronts continued
How Does Air Pressure Affect Weather?
Remember that air produces pressure. However, air
pressure is not always the same everywhere. Areas with
different pressures can cause changes in the weather.
These areas may have lower or higher air pressure than
their surroundings.
A cyclone is an area of the atmosphere that has lower
pressure than the surrounding air. The air in the cyclone
rises. As the air rises, it cools. Clouds can form and may
cause rainy or stormy weather.
An anticyclone is an area of the atmosphere that has
higher pressure than the surrounding air. Air in anticyclones
sinks and gets warmer. Its relative humidity decreases. This
warm, sinking air can bring dry, clear weather.
Cyclones and anticyclones can affect each other. Air
moving out from the center of an anticyclone moves
toward areas of low pressure. This movement can form a
cyclone. The figure below shows how cyclones and anticyclones can affect each other.
(IGH
PRESSURE
Anticyclone
,OW
PRESSURE
Critical Thinking
11. Compare Give two
differences between cyclones
and anticyclones.
TAKE A LOOK
12. Identify In which
direction does air move: from
a cyclone to an anticyclone,
or from an anticyclone to a
cyclone?
Cyclone
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Section 2 Review
NSES
NSES ES 1j
SECTION VOCABULARY
air mass a large body of air throughout which
temperature and moisture content are similar
anticyclone the rotation of air around a high
pressure center in the direction opposite to
Earth’s rotation.
cyclone an area in the atmosphere that has
lower pressure than the surrounding areas and
has winds that spiral toward the center
front the boundary between air masses of
different densities and usually different
temperatures
1. Identify Relationships How are fronts and air masses related?
2. Compare Fill in the table to describe cyclones and anticyclones.
Name
Compared to surrounding air
pressure, the pressure in the
middle is...
cyclone
...lower than surrounding pressure.
anticyclone
What does the air
inside it do?
What kind of weather
does it cause?
sinks and warms
3. List What are four kinds of fronts?
4. Identify What are the source regions for the mT air masses that affect weather in
the United States?
5. Describe What kind of air mass causes hot, clear, dry summer weather in the
United States?
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Earth Science Answer Key continued
4. about 30 g/cm3
5. 10 g/m3 23 g/m3 0.43
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
5.
0.43 × 100 43% relative humidity
Humidity is the absolute amount of water
vapor in the air. Relative humidity is a measure
of how saturated with water vapor the air is.
wet-bulb and dry-bulb thermometers
about 87%
Water vapor condenses into liquid water.
The glasses are as cold as the air outside.
They absorb heat from the air inside. As a
result, the air inside cools below its dew
point. Water condenses on the glasses.
water vapor in the air
by shape and by altitude
A nimbostratus cloud is producing
precipitation.
water that falls to Earth from clouds
They get bigger.
several ice crystals joined together
when it is too heavy to be carried by
updrafts
How it affects weather
cP from northern
Canada
very cold winter weather and
cool, dry summer weather
mP from the North
Pacific Ocean
rain and snow in the Pacific
in the winter, fog in the
summer
mT from the Gulf of
Mexico
heat, humidity, hurricanes,
thunderstorms in East Coast
and Midwest
cT from the deserts
clear, dry, hot weather in the
summer
6. a place where two or more air masses meet
7. It rises because the cold air pushes it up.
8. a place where a warm air mass moves over a
9.
10.
11.
12.
Review
1. Possible answer: Water can condense from
cold air mass
The cold air masses push it out of the way.
Stationary fronts do not move, so the
weather they bring stays in one place.
Cyclones have lower pressures than surrounding areas, but anticyclones have higher
pressures. Cyclones bring rainy or stormy
weather, but anticyclones bring dry, clear
weather.
anticyclone to cyclone
Review
1. A front is a place where two air masses meet.
2. First row, from left to right: rises and cools;
air that is below its dew point.
2. the sun
3. Air rises and cools. When it cools below its
dew point, water vapor condenses to form
liquid water droplets or ice crystals. These
droplets or ice crystals form a cloud.
4. Sleet forms when liquid water freezes in clouds
and falls to the ground as ice. Snow forms
when water vapor turns directly into a solid.
5. Name
Altitude Shape
Precipitation?
cloudy, rainy, and stormy
Second row, from left to right: ... higher than
surrounding pressure; dry and clear
3. cold, warm, occluded, stationary
4. Pacific Ocean, Atlantic Ocean, Gulf of
Mexico
5. cT
SECTION 3 SEVERE WEATHER
Cirro-stratus
high
layered
no
1. It must contain enough water vapor to form
Altocumulus
middle
puffy
no
Nimbostratus
low
layered
yes
Cumulonimbus
low to
middle
puffy
yes
clouds.
Electricity travels between an area with a
positive charge and an area with a negative
charge.
sound produced by vibrations in expanding
air that was superheated by lightning
The center of the tornado has low pressure.
when it touches the ground
74 mph to 112 mph
Ocean water is too cold there.
the Coriolis effect
the center of the hurricane that contains
warm, fairly calm air with low pressure
2.
3.
4.
5.
6.
7.
8.
9.
SECTION 2 AIR MASSES AND FRONTS
1.
2.
3.
4.
Air mass
by water content and temperature of the air
dry, warm
northern Canada
Warm air masses form over warm water or
land, and the North Atlantic and Pacific are
cold.
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Earth Science