Weather Patterns
Paula Messina
1. Planetary Wind Belts
2. Air Masses and Fronts
Planetary Wind Belts (1)
• Maximum insolation
@ Equator
• Hot air rises
• Minimum Insolation
@ Poles
• Cold air sinks
• Effect: a general circulation from
Equator to Poles
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Planetary Wind Belts (2)
• Where air rises, it expands, it cools down, it produces clouds
• The Equator, therefore, is a zone of
LOW PRESSURE
• (Low pressure is associated with clouds/rain, high temperatures)
Planetary Wind Belts (3)
• Where air sinks, it becomes compressed.
• Any liquid water in it evaporates, causing the air to be very dry.
• The Poles, therefore, are zones of HIGH
PRESSURE
• (High pressure is associated with cold, dry air)
Planetary Wind Belts (4)
90 o N
• Air rising at the
Equator also sinks at latitudes of 30 o N and 30 o S.
• Therefore, in addition to the
Poles, there are high pressure zones of sinking air found at
30 o N and 30 o S.
90 o S
30 o N
30 o S
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Planetary Wind Belts (5)
• Air rises also at latitudes of of
60 o N and 60 o S.
• Therefore, in addition to the
Equator, there are low pressure zones of rising air found at
60 o N and 60 o S.
60 o N
0 o
60 o S
Planetary Winds (6)
90 o N n So, Earth is divided into zones, each 30 o of latitude thick, of alternating high and low pressure
90 o S
60 o N
30 o N
0 o
30 o S
60 o S
Planetary Winds (7)
90 o N n Remember that air moves from areas of high pressure to areas of low pressure!
60 o N
30 o N
0 o
30 o S
60 o S
90 o S
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Planetary Winds (8)
90 o N n n
Remember the
Coriolis Effect!
Air is deflected to the right in the
Northern
Hemispehere, and is deflected to the left in the Southern
Hemisphere!
90 o S
60 o N
30 o N
0 o
30 o S
60 o S
Planetary Wind Belts (9)
• Planetary Wind Belts
• Polar easterlies
• Prevailing westerlies
• Tradewinds
• Zones of Convergence
(and Low Pressure)
• 0 o , 60 o N, 60 o S
• Zones of Divergence
(and High Pressure)
• 30 o N, 90 o N, 30 o S, 90 o S
Planetary Wind Belts (10) n So, in the US, weather patterns migrate across the country generally from southwest to northeast
H
H
1004 mb
1008 mb
1012 mb
1016 mb
1020 mb
H
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Part 2: Air Masses and Fronts
What is an Air Mass?
n An air mass is a large quantity of air that has the temperature & humidity characteristics of the area over which it formed.
Cool and moist
Cool and dry
Warm and dry
Warm and moist
Air Mass Nomenclature n mP=maritime Polar n n cP=continental Polar n cT=continental Tropical mT=maritime Tropical mP cP cT mT
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n n
Air Mass Migrations
Air masses are driven by planetary winds
Air masses may travel at slightly different speeds, and in slightly different directions cP mP mP cP mP mP mP cT cTcT mT
Fronts n When air masses collide, a front forms.
mP cT
Cold Fronts (1) mP cT n When a cold air mass bumps into a warm air mass, a cold front forms at the boundary.
n In cross-section, it looks like this:
Cold front
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Cold Fronts (2) mP cT n Cold air wedges its way under warm air
(because cold air is denser) n This causes the warm air to rise
Cold front
Cold Fronts (3) mP cT n When air rises, it expands n When air expands, it cools down n When it cools to its dewpoint, clouds form
Cold front n n n
Cold Fronts (4) mP cT
Cold fronts are associated with thick (“cumuloform”) clouds, forming from rapidly rising air
Cold fronts produce heavy precipitation for short periods of time
Lightning is possible
Cold front
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Warm Fronts (1) n When a warm air mass bumps into a cold air mass, a warm front forms at the boundary.
cT cP cT cP
Warm Fronts (2) n In cross section, this is what a warm front looks like:
Warm front n cT
Warm Fronts (3)
Since the cold air mass is denser, it stays close to the ground.
cP n n
The warm air mass gently slides over the cold air mass
So air is rising, but not significantly, as in the case of a cold front
Warm front
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cT cP
Warm Fronts (4) n The clouds that form are relatively flat
(“stratiform”) clouds n Stratiform clouds are associated with light rain or drizzle, for long periods of time.
Warm front
Cold Fronts vs. Warm Fronts n Cold fronts tend to move rapidly, so heavy precipitation or showers last for short bursts of time n Warm fronts move more slowly—and their boundaries span more miles—so light rain may persist for several days.
Occluded Fronts (1) mP cP cT n A warm air mass may get “sandwiched between” two cold air masses
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mP cT cP
Occluded Fronts (2) n Two cold air masses force a warm air mass totally off the ground
Occluded front mP cT cP
Occluded Fronts (3) n Clouds and precipitation are similar to those of a warm front, and a cold front in succession
Occluded front
Stationary Front n A stationary front may be any type of front
(cold, warm, or occluded) where the air masses stop moving n Since the front does not move, precipitation may persist for a long period of time.
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Weather Map Symbology (1) mP cT cP cT n Cold front (west); warm front (east)
Weather Map Symbology (2) mP mP cT n Occluded front (west); stationary front (east)
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