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Atmospheric pressure:
– force exerted by a column of air per unit area
– Normal atmospheric pressure at sea level =
1013 millibars
1.Temperature changes:
• When air is heated:– air expands and PRESSURE DROPS
• When air is cooled:– air compresses and PRESSURE
INCREASES
2. Rotation of earth:
• Earth’s rotation causes air to accumulate in certain latitudes and to be deflected away from certain latitudes
• accumulation : HIGH pressure• deflection: LOW pressure
Highs and Lows in cross-section:
• HIGHS:– clear skies
• rising barometer means good weather
• LOWS:– cloudy skies
• falling barometer means bad weather
Equatorial Low
• 5oN - 5oS• Intertropical Convergence
Zone (ITCZ)• thermal Low
– high sun angles, long days, available energy
– ascending air– heavy precipitation– cloud cover
Subtropical Highs• 25o - 40o N & S• rotation-induced Highs• air deflected to subtropics• descending air• clear skies• hot dry air• great deserts here
Subpolar Lows
• 55o - 70o N & S• rotation-induced Lows• warm air from low latitudes is lifted
as it meets cold polar air• ascending air• storm centers here
Polar Highs
• 90o N & S• thermal Highs• cold polar temps at high latitudes• descending air
Note: all pressure belts shift seasonally
What causes wind?
Wind is air moving from High to Low pressure.
Wind is named after direction it comes FROM.
(a “west wind” comes out of the west; flows eastward)
1.Wind Speed is determined by:
a. Steepness of pressure gradient• Steep gradient: closely spaced isobars• Gradual gradient: widely spaced isobars
b. Friction• Friction from surface lowers wind speed
b. Coriolis force
• apparent deflection of moving things (like the wind) on a rotating surface (like the earth)
• Imagine tossing a ball across a rotating room…
Ball appears to be deflected to the right, but it has been going in the same direction all along.
the ball’sdirection
Airplanes, rockets, migrating birds, ocean currents, air are deflected from their paths of motion because the earth is rotating.
in Northern Hemisphere, deflection to RIGHT of movement
in Southern Hemisphere, deflection to LEFT of movement
Watch this animation…
Deflection increases with latitude:
• no Coriolis at equator; • greatest deflection at poles
• Imagine sitting on a chair on a platform at varying latitudes….
If you are sitting on the north
pole, how many degrees will the room rotate/spin
in one day?
YOU! 360°
If you are between the poles and the equator,
how many degrees will the room rotate/spin in one
day?
Between 0 and 360, depending on latitude
c. Friction
the “drag” produced by earth’s surface
– applied opposite direction of motion
– reduce angle of Coriolis deflection
Winds in Upper Atmosphere
no friction
only the pressure gradient and Coriolis effect
– wind is parallel to isobars: GEOSTROPHIC WIND
Winds Aloft
• Upper Level Westerlies (25o - 90o)
• Polar Low• Tropical High
Pressure Belt (15o - 20o N & S)
• Equatorial Easterlies
Jet Streams
• Narrow zones of extremely high wind speeds
• occur where there are strong temp contrasts
• Polar Jet (westerly)• Subtropical Jet
(westerly)