GEOG 1112: Weather and Climate Atmospheric Pressure, Wind, & Global Circulation.
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Transcript of GEOG 1112: Weather and Climate Atmospheric Pressure, Wind, & Global Circulation.
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GEOG 1112: Weather and Climate
Atmospheric Pressure, Wind, & Global Circulation
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Air Pressure
• Weight of air exerting pressure on surface of Earth
• Air pressure decreases with altitude
• Warm air is lighter than cool air, so it tends to rise – cool air sinks
Vertical and horizontal pressure differences that lead to
atmospheric motions.
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Air Pressure & Altitude
Lower Density,Lower Pressure
Higher Density,Higher Pressure
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Pattern of pressure: a non-linear decrease with height
Why?
Vertical Changes in Pressure
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Atmospheric Pressure Systems
High-pressure system-also called anticyclone-circulating body of air-descending air-clockwise circulation innorthern hemisphere
Low-pressure system-also called cyclone-circulating body of air-rising air-counterclockwise circ. in northern hemisphere
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Atmospheric Pressure Map
Isobars – lines of equal air pressure on a map
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Direction of Air Flow• Unequal heating of land surfaces
• Pressure gradient force – air flows from high to low
• Coriolis force – deflection or change in direction caused by Earth’s rotation
• Frictional forces – places a drag on that air flow
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Isobars and Pressure Gradient
• The closeness of isobars decides the gradient between high and low pressure
• The closer the isobar, the higher the winds and vice versa
• Winds at the surface always blow at right angles to isobars
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Pressure Gradient
H L
isobars
1028
mb
1024
mb
1020
mb
1016
mb
1012
mb
1008
mb
1004
mb
1000
mb
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Pressure Gradient Force
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Coriolis Force•Due to Earth’s rotation
•Pulls wind to right in northern hemisphere
•Pulls wind to left in southern hemisphere
•Strongest at poles
•None at equator
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Geostrophic Wind• Horizontal wind in the upper atmosphere that
moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.
Low Pressure
High Pressure
500 mb
504 mb
508 mb
512 mbPGF
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Frictional Forces
• Near surface, friction (F) works against pressure gradient force (PGF), so resulting wind direction is between pressure gradient force and coriolis force (CF)
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Three Forces Combined
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Rossby Waves
Figure 6.17
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Jet Stream
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Strong boundaries often occur between warm and cold air. In
the mid-latitudes, the polar front marks this thermal discontinuity
at the surface.
The Polar Front and Jet Streams
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L L
H
H
Global Pressure & Atmospheric Circulation
• Unequal heating of tropics and poles
• Global circulation on non-rotating, uniform Earth
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Global Circulation Model
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Seasonal Changes in Circulation• ITCZ migrates with subsolar point
• Trade winds, STHs, westerlies, all follow the ITCZ north & south with seasons
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Seasonal Changes in Circulation
• Monsoon
– Seasonal shift of prevailing wind dir. due to land/water contrasts
– Happens throughout subtropical regions
– Most significant in South & SE Asia
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South Asian MonsoonWinter Monsoon•Cold air over Asia
•Sinking air/High pressure
•Cool, dry NE winds
•ITCZ far south
Summer Monsoon•Warm air over Asia
•Rising air/Low pressure
•Warm, moist wind – rain
•ITCZ swings north
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Local Wind SystemsSea Breeze•Sun-heated air over land rises – Low pressure
•Cooler air over water sinks – High pressure
Land Breeze•Night air over land cools fast – High pressure
•Air over water stays warmer – Low pressure
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Local Wind Systems – Mountain-Valley Breeze
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Subtropical High-pressure Cells
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Global Oceanic Circulation
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Oceanic Circulation• Thermohaline Circulation – warm water in gulf stream
cools and evaporates as it heads north making it heavier – downwelling, then a long trip to the Pacific where upwelling occurs
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ENSONormal El Niño