Download Air Masses and Fronts

Air Masses and
Fronts
Notes
Types of Air Masses
• An air mass is a huge body of air. The
air at any given height is an air mass
and has about the same temperature,
humidity, and air pressure.
• North America’s weather is influenced
by four types of air masses.
Types of Air Masses
• A tropical air mass forms in the tropics. A
tropical air mass has warm air.
• A polar air mass forms near the poles. A
polar air mass has cold air.
• A maritime air mass forms over the ocean
and has humid air.
• A continental air mass forms over land
and has dry air.
Types of Air Masses
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A maritime tropical (mT) air mass has
warm, humid air. (*impacts Mississippi)
A maritime polar (mP) air mass has cold,
humid air.
A continental tropical (cT) air mass has
hot, dry air.
A continental polar (cP) air mass has
cold, dry air.
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How Air Masses
Move
These air masses move. Global winds
usually move these masses of air from one
part of the globe to another.
The major wind belts over the United States
are the westerlies. These winds usually
push air masses from the west to the east
across the States.
Jet streams also push air masses in the
same west-to-east direction.
How Air Masses
Move
• As these air masses move, they will
eventually collide with one another. A
front is a boundary where two air
masses meet.
• Storms and other types of weather
develop along frontal boundaries.
Types of Fronts
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A cold front forms when a cold air mass slides
under a warm air mass, forcing the warm air up
very quickly.
Remember that warm air will rise up over the cold
air because the warm air is less dense.
Thunderstorms can occur with cold fronts.
Precipitation will usually occur behind the front.
A cold front on a weather map is indicated by a
blue line with triangles pointed in the direction that
the front is moving towards.
Types of Fronts
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A warm front forms when a warm air mass
moves over a cooler air mass.
This lifting is generally much more smooth and
slow than a cold front.
Light rain or snow may fall.
Precipitation will usually occur ahead of the
front.
On a weather map, this would appear as a red
line with semi-circles indicating the direction of
movement.
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Types of Fronts
Sometimes cold and warm air masses meet,
but neither air mass moves over or under the
other, they may even travel parallel to the
frontal boundary. This is known as a
stationary front.
A stationary front can bring many days of
clouds and precipitation.
A stationary front on a weather map is
indicated by inverting blue/red
triangles/semicircles.
Types of Fronts
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A warm air mass sometimes is caught between
two cooler air masses. OR a cold front catches
up to a slower-moving warm front. This forms
an occluded front.
In an occluded front, the warm air mass is cut
off from the ground as it is lifted above the two
other air masses. The weather may turn cloudy,
and rain or snow may fall.
These are shown as purple lines of alternating
triangles and semicircles.
Step 1
Step 2
Types of Fronts
• In some cases, one front can eventually
turn into another. And in the strongest
cases, this will turn into a mid-latitude
cyclone, as the westerlies, the Coriolis
effect, and rising air all combine into a
severe weather system.
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Cyclones and
Anticyclones
A cyclone is a swirling center of low air pressure.
The winds in a cyclone spiral around a center. In a
cyclone, winds spiral inwards towards the lowpressure center.
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The Coriolis effect makes winds spin
counterclockwise in a Northern Hemisphere cyclone.
Air spins upward in a cyclone due to the airs
converging at the base. The result is lifting air that
will condense into clouds, extreme wind shifts, and
precipitation.
Cyclones play a large part in weather of the United
States.
Cyclones and
Anticyclones
• An anticyclone
is a high-pressure center of
dry air. An anticyclone is the opposite of a
cyclone. In an anticyclone, winds spin
outward away from the high-pressure
center.
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The Coriolis effect makes winds spin
clockwise in a Northern Hemisphere
anticyclone.
The air spins downward in an anticyclone
due to the divergence of air at the base. The
result is dry, clear weather.
High Pressure
System
• A high pressure system is a whirling
mass of cool, dry air that generally
brings fair weather and light winds.
When viewed from above, winds spiral
out of a high-pressure center in a
clockwise rotation in the Northern
Hemisphere. These bring sunny skies.
Low Pressure
System
• A low pressure system is a whirling
mass of warm, moist air that generally
brings stormy weather with strong
winds. When viewed from above, winds
spiral into a low-pressure center in a
counterclockwise rotation in the
Northern Hemisphere. A low pressure
system is represented as a big, red L.