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Chapter 3
Section 2 Air Masses and Fronts
Objectives
• Identify the four kinds of air masses that influence
weather in the United States.
• Describe the four major types of fronts.
• Explain how fronts cause weather changes.
• Explain how cyclones and anticyclones affect the
weather.
Chapter 3
Section 2 Air Masses and Fronts
Air Masses
• Changes in weather are caused by the movement
and interaction of air masses.
• An air mass is a large body of air where temperature
and moisture content are constant throughout.
• Cold Air Masses Most of the cold winter weather in
the United States is influenced by three air masses
formed over the polar region.
Chapter 3
Section 2 Air Masses and Fronts
Air Masses, continued
Cold and wet
Cold and dry
Cold and wet
Chapter 3
Section 2 Air Masses and Fronts
Air Masses, continued
• Warm Air Masses Three warm air masses influence
the weather in the United States.
Chapter 3
Section 2 Air Masses and Fronts
Air Masses, continued
Warm and wet Warm and dry
Warm and wet
Chapter 3
Section 2 Air Masses and Fronts
Fronts
• Front - the area in which two types of air masses
meet
Chapter 3
Section 2 Air Masses and Fronts
Cold Front - where cold air moves under warm air,
which is less dense, and pushes the warm air up.
Chapter 3
Section 2 Air Masses and Fronts
Warm Fronts - where warm air moves over cold,
denser air.
Chapter 3
Section 2 Air Masses and Fronts
Occluded Front - forms when a warm air mass is
caught between two colder air masses. This front has
cool temperatures and large amounts of rain and snow
Chapter 3
Section 2 Air Masses and Fronts
Stationary Front - forms when a cold air mass meets
a warm air mass, often brings many days of cloudy,
wet weather
Chapter 3
Section 2 Air Masses and Fronts
Air Pressure and Weather
• Cyclones are areas that have lower pressure than
the surrounding areas do. Air moves in and rises.
•Air moves counter-clockwise, toward the center.
• Anticyclones are areas that have high pressure.
•Air moves clockwise, away from the center.
• Strong winds can be whipped between the two areas.
Facts you will
need to know
for the quiz:
Cold Front
1. Cold pushes under warm
2. Cumulous clouds
3. Stormy, cloudy weather
4. High winds
5. Temperature changes
from warm to cold
Warm front
1. Warm moves over cold
2. Stratus clouds
3. Clouds form, possibly storms
4. Light winds
5. Temperature changes from
cold to warm
Occluded front
1. Warm is caught between 2
cold, warm is pushed up
2. Cumulonimbus clouds
3. Very stormy and cloudy
4. Strong winds, then lighter
winds
5. Temperature changes from
cold to warm to cold
Stationary front
1. Warm pushes against cold,
both are equally strong
2. Nimbostratus clouds
3. Many days of clouds,
possibly rain
4. Light winds
5. Temperature does not
change
M = maritime
Air mass is formed over water,
is has high humidity (it is wet).
C = continental
Air mass is formed over land,
is has low humidity (it is dry).
P = polar
Air mass is formed in the north,
is has lower temperature.
T = tropical
Air mass is formed in the south,
is has higher temperature.
What is weather?
1
Precipitation
• Water falling from clouds is called
precipitation.
• Precipitation occurs when cloud droplets
combine and grow large enough to fall
to Earth, 0.2 mm.
• That is 100 times larger than the tiny “misty”
droplets that make up a cloud.
• The water needs a “seed”, a speck of dust or
salt on which to attach.
What is weather?
1
Precipitation
• The size of raindrops depends on several
factors. One factor is the strength of updrafts
in a cloud, which can keep drops suspended
in the air where they can grow larger.
• The rate of evaporation as a drop falls to
Earth also can affect its size.
What is weather?
1
Precipitation
• Air
temperature
determines
whether water
forms rain,
snow, sleet, or
hail—the four
main types of
precipitation.
What is weather?
1
Precipitation
• Drops of water
falling in
temperatures above
freezing fall as rain.
• Snow forms when
the air temperature
is below freezing.
Snow is formed
when water goes
directly from vapor
to solid forming
unique 6-sided
shapes.
What is weather?
1
Precipitation
• Sleet forms when
raindrops pass through a
layer of warm air, melts,
and then refreezes near
Earth’s surface, forming
ice pellets.
What is weather?
Hail
1
• Hail is precipitation in the
form of lumps of ice.
• Hail forms in
cumulonimbus clouds of a
thunderstorm when water
freezes in layers around a
small nucleus of ice.
• They are tossed up and
down by rising and falling
convection currents.
• Hail may be the most dangerous form of
precipitation.