Geography 101 The Physical Environment
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Atmospheric Circulation

Air Pressure

Air pressure is the force exerted by the weight of a column of air above a particular location.

• Average sea level pressure = 1013.2 millibars

• High Pressure

• Low Pressure

Follow link to "Barometric Pressure" from USA Today

Cause of pressure variations

• Mechanical

• Thermal

Wind

• Wind - movement of air molecules from one place to another.

• Pressure gradient - change in pressure over distance. Go to animation

• Coriolis effect - effect of earth rotation on the path and direction of wind. Go to animation

• Surface friction

Cyclones and Anticyclones

Cyclones and Troughs

• Cyclone - area of low pressure; trough - elongate area of low pressure.

• Counterclockwise rotation of wind in Northern Hemisphere / clockwise in Southern Hemisphere

• Convergence at center at the surface; divergence aloft

Anticyclones and ridges

• Anticyclone - area of high pressure; ridge - elongate area of high pressure.

• Clockwise rotation of wind in Northern Hemisphere / counterclockwise in Southern Hemisphere

• Divergence from center at the surface; convergence aloft

Follow link to Focus on Circulation Around Lows and Highs

Local and Regional Scale Wind

Local scale winds

• Land - Sea breeze - Thermally driven circulation Go to animation

• Mountain - Valley winds - Thermally driven circulation; daytime valley wind (up slope flow) - evening mountain wind (down slope flow).

• Chinook - warm, dry wind on the leeward side of mountains.

Regional scale winds

• Monsoon circulation  - winds that change direction with season - thermally driven circulation.

• Follow link to Focus on Monsoons

Continue to "Global Circulation"        

Can you....

• .. describe the factors that determine wind direction and speed at the surface of the earth?

• .. draw a diagram of high and low pressure systems with the proper circulation patterns around them as they appear in the Northern Hemisphere?

• .. describe how land sea/breezes form?

Further Reading (optional)

• What's happening in areas of high and low pressure (USA Today)

• Understanding air density (USA Today)