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Fronts
2010
Air Masses
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Air masses: a huge
body of air with
similar temperature,
humidity, and
pressure.
In America, we have
four air masses that
affect us. They are
based on
temperature and
humidity.
Types of Air Masses
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Tropical: form over the
tropics, so they are
warm and will have low
air pressure.
Polar: form north/south
of 50º latitude. So they
are cold and will have
high pressure.
Maritime: form over the
oceans so they are
humid.
Continental: form over
the middle of continents
(land) so they are dry.
Air Masses Compared
Type
Maritime
tropical
(mT)
Maritime
polar (mP)
Temp
Humidity
Pressure
Warm
Humid
Low
Cold
Humid
High
Continental Warm
tropical (cT)
Dry
Low
Continental
polar (cP)
Dry
high
Cold
Air Masses and Global Winds
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In America air masses are pushed by the
prevailing westerlies.
So they tend to move west to east.
Weather Fronts
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Fronts: where two
air masses meet and
don’t mix.
Front is a military
term where two
armies meet to
fight.
The collision of
fronts often causes
storms and
changeable weather.
There are four
kinds of fronts.
Cold Fronts
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Cold fronts have cold,
dense air.
They will slide under
warm, less dense air.
So the warm air is
pushed up, which
causes it to cool.
This means it can hold
less water vapor, so
clouds form as it
reaches its dew point.
This can cause rain and
storms behind the
front.
They move quickly, and
bring cool, dry air
after the front passes.
Warm Fronts
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Moving warm air collides with cold air.
Tend to move more slowly than cold fronts.
Can bring cloudy skies, showers, and light rain
preceding the front.
Weather will be warm and humid after it
passes.
Stationary Fronts
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Occur when cold and
warm fronts meet
and neither is strong
enough to force the
other to move.
It is a “standoff.”
The warm air will
condense and form
rain, fog, snow, or
clouds.
Can cause many days
of wet weather.
Occluded Fronts
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Occur when a warm air
mass is trapped
between two cold air
masses.
The warm air is pushed
up when the two cold
air masses meet.
It is cut off from the
ground, or occluded.
Tend to produce strong
thunderstorms.
Weather Map Symbols
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These are the
symbols you
would see on a
weather map.
The arrows or
bumps point in
the direction
the front is
moving.
Low Pressure Areas
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Low pressure areas will
have warm, rising air.
Cool air around it will
spiral in counter
clockwise (Coriolis
effect).
As the air rises, it cools
and reaches its dew
point.
So clouds and rain will
form.
Called cyclones from a
Greek word meaning
wheel.
High Pressure Areas
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High pressure centers of
dry air.
Spiral outward towards
areas of lower pressure.
Coriolis effect causes
them to spiral clockwise
in the Northern
Hemisphere.
Made of cool, falling air,
so its relative humidity
drops.
Produces clear, dry
weather.
Called anticyclones.