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 in Scientific units are millibars –Sea level pressure mb Highest pressure was mb (32.01 in) Lowest pressure was 870 mb (25.69 in) (prior to 2005 hurricane season)

AD D B C C In the beginning - a side view of atmosphere 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

AD D B C C HOTCOLD HIGHLOW Heat Earth at A 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 As air rises it expands. Expanding air cools adiabatic cooling

AD D B C C HOTCOLD LOW HIGH LOW Upper troposphere winds 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

AD D B C C HOTCOLD LOW HIGH LOW 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 Sinking air is compressed Becomes warmer adiabatic warming

AD D B C C HOTCOLD LOW HIGH LOW Conclusions 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

AD D B C C HOTCOLD LOW HIGH LOW Conclusions 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

AD D B C C HOTCOLD LOW HIGH LOW Conclusions Upper level winds blow in opposite directions from lower level winds. Upper level pressure systems are the opposite of lower level pressure systems.