Download Air Masses and Fronts

Previewing the Chapter
• Read the titles of the chapter and each
section
For Sections 1 and 4:
• Read the headings (the words in green)
• Scan the illustrations, charts, tables, and
graphs
For Sections 2 and 3:
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• CREATE YOUR OWN TITLE PAGE AS
YOU DO THIS.
Air Masses and Fronts
Section One
Pages 76-82
What is an air mass?
Air mass = a huge body of air that has
similar temperature, humidity, and air
pressure throughout
A single air mass may spread over an area
of millions of square kilometers and be up
to 10 kilometers high!
Scientists classify air masses according
to two characteristics:
1. temperature
2. humidity
Temperature
Whether an air mass is warm or
cold depends on the temperature
of the region over which the air
mass forms.
Tropical Air Mass
• Tropical, or warm air masses form in the
tropics.
• They have low air pressure.
What are the tropics?
• Regions of the Earth that lie roughly in the
middle of the globe. The tropics are between
the latitude lines of the Tropic of Cancer and
the Tropic of Capricorn. The tropics are warm all
year, averaging 77-82 degrees Fahrenheit.
Polar Air Mass
•
•
•
Polar, or cold, air masses form near the
poles.
They have high air pressure.
What are the polar regions?
The ice-covered areas around the far south
and north ends of the globe, the North and
South Pole. The continental regions of Arctic
and Antarctica settle in these two polar
coordinates respectively. The weather is very
cold, far below freezing temperatures, all year
around.
Humidity
Whether an air mass is humid or dry
depends on whether it forms over water or
land.
Maritime Air Mass
• Maritime air masses form over oceans.
• Water evaporates from the oceans which
causes the air to become very humid.
Continental Air Mass
• Continental air masses form over land, in
the middle of continents.
• They are very dry.
North American Air Masses
Four major types of air masses influence the
weather in North America:
1. Maritime Tropical (mT)
2. Maritime Polar (mP)
3. Continental Tropical (cT)
4. Continental Polar (cP)
Maritime Tropical
• They form over the ocean.
• They are warm and wet.
• They influence weather in the central and
eastern United States and also along the
West Coast.
• In the summer they bring hot, humid
weather. In winter, a humid air mass can
bring heavy rain or snow.
Maritime Polar
• They form over the ocean.
• They are cool and wet.
• Affect the West Coast more than the East
Coast.
• In the summer these masses bring fog,
rain, and cool temps to the west coast.
Continental Tropical
• They form over land.
• They are hot and dry.
• Form only in summer over dry areas of the
Southwest and northern Mexico.
• Cover a smaller area.
• Occasionally they move NE bringing hot,
dry weather to the southern Great Plains.
Continental Polar
• They form over land, central and northern
Canada and Alaska.
• They bring cool or cold air.
• In winter, they bring clear, cold, dry air to
much of North America.
Discover Activity
With your group members, read the
steps in the Discover Activity (76).
 With your group members, make a
prediction about what will happen when
the divider is removed (#4).
 Now write a hypothesis stating what
would happen if a mass of cold air ran
into a mass of warm air. With your
group members of course.

How Air Masses Move
The prevailing westerlies are
the major wind belts in the
continental United States.
They generally push air
masses from the west to the
east.
Fronts
Fronts are the area where air masses meet
and do not mix.
 Why don’t they mix? Think about the red
and blue water…
 When air masses of different temperatures
and densities collide, the collision causes
storms and changeable weather.

Types of Fronts
There are 4 types of fronts:
1. Cold fronts
2. Warm fronts
3. Stationary fronts
4. Occluded fronts
The kind of front that develops depends on
the characteristics of the air masses and
how they are moving.
Cold Fronts
A cold front forms when cold air moves
underneath warm air, forcing the warm air to
rise.
 Cold fronts move quickly, so they can cause
abrupt weather changes, including violent
thunderstorms.

Warm Fronts
A
warm front forms when warm air moves
over cold air. The warm air mass is
moving faster than the cold air mass.
 Clouds, storms, and rain accompany
warm fronts.

http://www.youtube.com/watch?v=huKYKykjcm0
Stationary Fronts
When two air masses meet but neither one
has enough force to move the other.
 Could bring many days of clouds and
precipitation.


http://www.youtube.com/watch?v=hFAkqHyFtkA
Occluded Fronts
When a cold air mass and a cool air mass
come together, the warm air caught between
them is forced upward.
 The two cooler air masses may mix while the
warm air mass is cut off, or occluded, from
the ground.


https://www.youtube.com/watch?v=Mfm8H8asRa4
Cyclones
A swirling center of low air pressure is called
a cyclone.
 On a weather map they are marked with an L,
for low pressure.
 Winds spiral inward toward the center of the
system.

Cyclones
Cyclones impact weather in the United
States.
 Cyclones and decreasing air pressure are
associated with storms and precipitation.

Anticyclones
High-pressure centers of dry air
 On a weather map they are marked with an H,
for high pressure.
 Winds spin clockwise in the Northern
Hemisphere.
 Cause dry, clear weather.

Words Worth Knowing (ISN #29)
Air mass
 Tropical
 Polar
 Maritime
 Continental

Front
 Occluded
 Cyclone
 Anticyclone

Bill Nye – Storms
https://vimeo.com/124214502
 23:00
