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Fronts, Highs, and Lows
Science 10
Weather Dynamics
Fronts
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Air masses eventually move around due
to worldwide wind currents, and
overtake one another.
The zone between the air masses is
called a front.
Meteorologists define four general types
of fronts: cold, warm, stationary, and
occluded.
Cold Fronts
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A moving cold air mass overtakes a warmer
air mass.
Because the cold air is more dense than the
warm air, it moves in below the warm air and
pushes the warm air upwards.
This causes a cumulus or cumulonimbus
cloud to form.
Rain, thunderstorms, and tornadoes result
from cold fronts.
Cold Fronts (cont’d)
Warm Fronts
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A warm air mass overtakes a cooler one.
The warm air gently rides up over the trailing
end of the cold air mass.
Stratus and nimbostratus clouds form and can
bring light rain or snow lasting longer than it
does with a cold front.
After the front passes, it leaves warmer
temperatures than before it arrived.
Warm Fronts (cont’d)
Stationary Fronts
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Cold and warm air masses are in
contact, but neither is moving.
Warm air drifts up and condenses to
form stratus clouds and often rain.
The front may remain stationary for
several days.
The result is a dreary, gloomy sky.
Occluded Fronts
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A cold air mass catches up with a
slower-moving warm front.
Many types of clouds are present, one
after the other.
Occluded fronts often cause a period of
steady precipitation.
Occluded Fronts (cont’d)
High Pressure Systems
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Air descends and exerts more pressure on the
region below.
When an air mass forms over cold ground,
the air cools, becomes more dense, and
settles lower to the ground. This creates
more pressure, so the system draws more air
in from above. Air at the bottom is forced
out and to the right, due to the Coriolis
effect.
High Pressure Systems
(cont’d)
High Pressure Systems
(cont’d)
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The overall result is a clockwise rotation
of air called an anticyclone.
Because the cool descending air has
little moisture, high pressure systems
create a clear sky.
They may be hundreds of km across.
They cause weather conditions to stay
the same for several days.
Low Pressure Systems
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Very intense heating of the ground can create
a low pressure system.
The heated air rises leaving less dense air
below.
As the air rises, it pulls more air beneath it.
The Coriolis effect causes the air to curve to
the right.
This results in a counterclockwise rotation of
air known as a cyclone.
Low Pressure Systems (cont’d)
Low Pressure Systems (cont’d)
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Lows are generally smaller in size than
highs.
They bring unstable conditions, which
cause changing weather, precipitation,
and often storms.