WHY AND HOW DOES THE WIND BLOW?
W. Mitchell Sams, Jr.
THE EARTH DOES NOT LIKE EXTREMES
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Temperature differences make pressure differences and pressure
differences drive the wind
So wind is attempting to correct the temperature differences that gave rise
to pressure differences in the first place
Once moving, wind tends to spin
RADIATION
• Most radiation produced by sun that reaches earth is in the visible range
• This radiation absorbed by earth
• And re-emitted as infrared (heat)
• Which heats the air above it
ELECTROMAGNETIC SPECTRUM
COMPOSITION OF AIR
Permanent
• Nitrogen
• Oxygen
• Argon
• Others
78%
21%
0.93%
<0.1%
Variable
• Water vapor
0-4%
• Carbon dioxide
0.038%
• Methane, NO2, Ozone
WEIGHT OF AIR
• A column of air 1 inch square stretching to top of atmosphere weighs 14.7
pounds
• When more molecules packed into column it becomes more dense,
weighs more and surface pressure goes up
• Pressure decreases rapidly with height
AIR PRESSURE AND MOVEMENT
• Air moves from high pressure to low pressure, creating wind
• Any air that rises will expand and cool
• Any air that sinks is compressed and warms
THE ATMOSPERE OBEYS THE GAS LAW
Pressure = temperature x density x constant
VAPOR SUPPLY/VAPOR CAPACITY
Warm air holds more moisture than cold air defined as vapor capacity
(VC)
• Vapor supply (VS) is amount of water available. For example: Higher
vapor supply available over a body of water
• So relative humidity (RH) is higher in cold air with same vapor supply as
warm air
LATENT HEAT
• The heat energy required to change a substance from one state to
another
• Thus, changing water into vapor (evaporation) is a cooling process
• Changing vapor into water (condensation) is a heating process
• The substance itself does not change temperature. But the environment
does.
THERMAL INERTIA
• Inertia is resistance to change
• Objects with high thermal inertia can absorb lots of heat without their
temperature changing appreciably (water)
• Objects with low thermal inertia absorb heat and temperature increases
(sand)
ISOBARS
HOW DOES WIND FORM?
• At equator more energy is absorbed than reflected
• At poles more energy is reflected than absorbed
• This is primary driver of atmospheric circulation
GLOBAL AIR CIRCULATION
FORCES ACTING ON WIND
1. Pressure Gradient Force (PGF)
2. Rotation of Earth (Coriolis Effect)
3. Friction
4. Centripetal/Centrifugal Forces
First Force Acting on Wind
PRESSURE GRADIENT (PGF)
• Pressure difference divided by distance
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Determines speed of wind
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On a weather map, the closer the isobars the faster the wind speed
Second Force Acting on Wind
CORIOLIS FORCE
• The force exerted on winds and oceans due to rotation of earth
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Is negligible at equator which is why hurricanes neither form nor cross
there
EFFECT OF PGF and CORIOLIS FORCE
Geostrophic Balance
Third Force Acting on Wind
FRICTION
Resistance to wind flow by the earth’s surface slows wind speed and may cause
change in direction toward the low pressure center
FORCES ACTING ON WIND
As wind slows due to friction, the Coriolis Force decreases
Fourth Force Acting on Wind
CENTRIPEDAL/CENTRIFUGAL FORCE
Gradient Wind
SANTA ANA WINDS
• As air is compressed by losing altitude it becomes hot
• Hot air can hold more moisture than cold
• So cold air at 50 deg F might have relative humidity (RH) of 100%
• But lower that air 2 miles and RH may be only 16% or less (because vapor
supply is same at both altitudes but vapor capacity is increased)
CLOUD TYPES
1. Cumulus – Heap or pile. Fair weather
2. Cirrus – Curl or tuft or wispy filament in the upper sky
3. Stratus – High layer of clouds stretched out like blankets
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Variant: Nimbus (or cumulonimbus) – Low, heavy layer with rain and
thunderheads
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Now recognize 10 genera and 26 species