What causes wind?
Single Convection Cell
Model for small scale circulation
By
Diana L. Duckworth
Rustburg High School
Campbell County, VA
Air Pressure Basics
• Air pressure is the weight of a column of air
• Depends on pull of gravity, mass of molecules
& kinetic energy of molecules (temperature
directly related when confined)
• Average weight of air on a 3 bedroom ranch
house is the equivalent of 1500 automobiles!
• Pressure exerted is equal in all directions
• Pressure decreases with altitude, air is less
dense at high altitude; fewer molecules above
Measurement of Air Pressure
• Barometer or barograph (records)
• Common units are inches of mercury
– Sea level pressure 29.92 in
• Scientific units are millibars
– Sea level pressure 1013.25 mb
• Highest pressure was 1083.8 mb (32.01 in)
• Lowest pressure was 870 mb (25.69 in)
(prior to 2005 hurricane season)
C
B
C
In the beginning - a side view of atmosphere
D
A
• Pressure at A = pressure at D
• Pressure B = pressure at C
• A & D pressures greater than C & D
pressures
• More molecules over A or D than C or B
D
LOW
C
HIGH
B
Heat Earth at A
D
COLD
LOW
C
As air rises it expands.
Expanding air cools
adiabatic cooling
A
D
HOT
COLD
• Air at A will be heated by Greenhouse Effect
• Air at A will be less dense, so it rises to B
• Thus air pressure at B increases compared to C
– more molecules at B than C
LOW
C
HIGH
B
LOW
C
LOW
A
HIGH
D
HOT
COLD
Upper troposphere
winds
HIGH
D
COLD
• Air will move horizontally from high to low
• From B to C called upper troposphere wind
• Makes pressure at D greater than A
– More molecules over D than over A
• As air moves from B to C it cools by radiation
LOW
C
HIGH
B
LOW
C
Sinking air is compressed
Becomes warmer
adiabatic warming
HIGH
D
COLD
LOW
A
HIGH
D
HOT
COLD
• Air flows from high at D to low at A
• Creates surface winds
• Cold dense air at C sinks to D, replacing air
that moved from D to A
• Completes convection cell
LOW
C
HIGH
B
LOW
C
LOW
A
HIGH
D
HOT
COLD
Conclusions
HIGH
D
COLD
• Vertical Motions caused by density differences
• Rising air creates a surface LOW and an upper
troposphere HIGH
• Sinking air creates a surface HIGH and an
upper troposphere LOW
LOW
C
HIGH
B
LOW
C
LOW
A
HIGH
D
HOT
COLD
Conclusions
HIGH
D
COLD
• Air moves horizontally (WIND) as a result of a
pressure gradient
• Moves from a high pressure to a low pressure
• Wind blows into a LOW - converges
• Wind blows out from a HIGH - diverges
LOW
C
HIGH
B
LOW
C
LOW
A
HIGH
D
HOT
COLD
Conclusions
HIGH
D
COLD
• Upper level winds blow in opposite
directions from lower level winds.
• Upper level pressure systems are the
opposite of lower level pressure systems.