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Low Pressure Systems
vs
High Pressure Systems
Let’s Compare
Low Pressure Systems (L)
High Pressure Systems (H)
Look!!! Convecting air…
LOW PRESSURE SYSTEMS
Low pressure systems involve air that tends to
rise, thus causing clouds and precipitation.
Those near the equator tend to be high energy
due to strong ground heating (convectional).
Low pressure systems may develop tornadoes
and sometimes develop into hurricanes, fuelled
by warm, very humid air evaporated from
tropical oceans in summer. They tend to be fast
moving, with plenty of cloud cover that
reduces temperature range, strong winds and
high rainfall.
LOW PRESSURE SYSTEMS
Hurricane off
USA fueled by
hot humid air
over the
Caribbean
LOW PRESSURE SYSTEMS
A convectional cumulonimbus cloud
results from strong ground heating at
the equator
LOW PRESSURE SYSTEMS
A tornado may develop from a
cumulonimbus cloud
HIGH PRESSURE
High Pressure means that air
tends to sink. Sinking air is
compressed, warms up as a
result and its relative humidity
falls below saturation. Any
clouds evaporate. Rainfall is
unlikely.
HIGH PRESSURE
LITTLE
WIND
CLEAR SKIES
High pressure systems have clear skies,
varying winds, little rainfall.
� Visibility is typically good
�Cloud cover is slight, resulting in a high
ranges of temperature (hot days, cold
nights).
�
FEW CLOUDS
CLEAR SKIES CAUSE FROST
VISIBLITY REMAINS BETTER IN
MOUNTAINS - LESS POLLUTION
Teacher
Page
Weather Fronts
Air Mass
• A large body
of air with
similar
temperature
and moisture.
• Air masses
form over
large land or
water masses.
MAP TAP 2002-2003
Weather Fronts
11
Frontal Boundaries
• Where air masses
interact – also
know as a front.
• 4 kinds of fronts:
–
–
–
–
Cold front
Warm front
Occluded front
Stationary front
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Weather Fronts
12
Cold Front
• A cold air mass
is replacing a
warmer air
mass.
• Shown on a
weather map
by a blue line
with triangles
pointing the
direction the
cool air is
moving.
Back to
activity
Cold Front
• Thunderheads
can form as
the moisture in
the warm air
mass rises,
cools, and
condenses. As
the front
moves
through, cool,
fair weather is
likely to follow.
Back to
activity
Warm Front
Back to
activity
• Warm air mass
•
replacing a
cooler air mass.
Shown on a
weather map
by a red line
with half circles
pointing the
direction the
warm air is
moving.
MAP TAP 2002-2003
Weather Fronts
15
Warm Front
Back to
activity
• As the warm air
mass rises, it
condenses into
a broad area of
clouds. A
warm front
brings gentle
rain or light
snow, followed
by warmer,
milder weather.
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Weather Fronts
16
Comparing Warm and Cold
Fronts
• Cold fronts move faster than warm fronts.
• The weather activity in a cold front is often
violent and happens directly at the front (after
the front has passed).
• Cold fronts have sudden gusty winds high in the
air creating turbulence.
• In a warm front the cloud formation is very low
(altitude wise) often creating situations of poor
visibility.
MAP TAP 2002-2003
Weather Fronts
17
Occluded Fronts
• When a warm
•
Back to
activity
front is trapped
by 2 cold fronts.
Shown on a
weather map by a
purple line with
alternating
triangles and
semicircles
pointing the
direction the front
is moving.
MAP TAP 2002-2003
Weather Fronts
18
Occluded Fronts
• The temperature
Back to
activity
drops as the
warm air mass is
occluded, or
“blocked,” from
the ground and
pushed upward.
Can bring strong
winds and heavy
precipitation.
MAP TAP 2002-2003
Weather Fronts
19
Stationary Fronts
Back to
activity
• A front that stops moving or is moving very
slowly.
• Shown on a weather map with alternating red
semicircles pointing away from the warm air
and blue triangles pointing away from the cold
air.
MAP TAP 2002-2003
Stationary Fronts
Back to
activity
• Where the warm and cold air meet, clouds and
fog form, and it may rain or snow. Can bring
many hours or days of clouds and precipitation.
MAP TAP 2002-2003
Locate the 4 types of fronts on this
weather map.
Hint
1
Hint
2
Hint
3
Hint
4
Cold Fronts
Warm Front
Stationary Front
Occluded Front
Be a Weather Forecaster
You are planning
to travel to
Alabama in 2
days. The high
temperature
there for today
is 68º F.
Use the map to help you predict whether the temperature in
Alabama will increase, decrease, or stay the same. Explain why
you think so.
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Be a Weather Forecaster
There is a cold front approaching. The
temperatures will probably be cooler
behind the front.
28
Be a Weather Forecaster
• Of course, meteorologists
(weather forecasters) use much
more data than fronts and air
masses to help them forecast
the weather more accurately.
But any forecast is just a
prediction of what might
happen. Even with the best
data, weather forecasts can be
wrong.
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Weather Fronts
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Summary
• Describe the 4 types of fronts and the
weather associated with each front.
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Weather Fronts
30