General Atmospheric Circulation
GENERAL ATMOSPHERIC CIRCULATION
A. PLANETARY PRESSURE BELTS
1. Equatorial calms (Doldrums) throughout the year a belt of low pressure surrounds the earth in the equatorial zone (except in the vicinity of the eastern Indian Ocean, East Indies and Australian region during the north hemisphere winter)
most of air motion is vertical with light and variable winds having a slight westward drift
2. Subtropical calms or subtropical highs (Horse latitudes) characterized by high pressure and relatively light winds or calms
the descending air that maintains the high pressure pattern is warmed adiabatically and develops a low relative humidity and clear skies
dryness of descending air is responsible for the prevalence of world’s greatest deserts either in or adjacent to the horse latitudes.
in the southern hemisphere, horse latitudes are mostly over water, so conditions are fairly uniform throughout the year.
in the northern hemisphere, a more drastic annual modification of the idealized pattern occurs following pronounced temperature variation of large areas relative to that in the oceans.
3. Subpolar low pressure belts (Stormy latitudes)
In the northern hemisphere, strong annual changes as pronounced temperature contrasts develop between land and water. In January, lows reverse to highs over land (forming Canadian and Siberian highs) but become extremely intense and stormy low pressure areas over the relatively warm North Atlantic and North pacific Oceans (Iceland low and Aleutian low)
In the southern hemisphere, little change occurs from summer to winter due to global encirclement by the southern oceans.
4. Polar high pressure belts (Polar highs)
On the average, high pressure areas exist over both polar regions, but intensities and locations of the centers are known to shift, being only rarely entered on the geographic poles.
B. PLANETARY WIND SYSTEMS
B-1. Surface Winds (winds in the lower troposphere)
1. Tradewinds
A. PLANETARY PRESSURE BELTS
1. Equatorial calms (Doldrums) throughout the year a belt of low pressure surrounds the earth in the equatorial zone (except in the vicinity of the eastern Indian Ocean, East Indies and Australian region during the north hemisphere winter)
most of air motion is vertical with light and variable winds having a slight westward drift
2. Subtropical calms or subtropical highs (Horse latitudes) characterized by high pressure and relatively light winds or calms
the descending air that maintains the high pressure pattern is warmed adiabatically and develops a low relative humidity and clear skies
dryness of descending air is responsible for the prevalence of world’s greatest deserts either in or adjacent to the horse latitudes.
in the southern hemisphere, horse latitudes are mostly over water, so conditions are fairly uniform throughout the year.
in the northern hemisphere, a more drastic annual modification of the idealized pattern occurs following pronounced temperature variation of large areas relative to that in the oceans.
3. Subpolar low pressure belts (Stormy latitudes)
In the northern hemisphere, strong annual changes as pronounced temperature contrasts develop between land and water. In January, lows reverse to highs over land (forming Canadian and Siberian highs) but become extremely intense and stormy low pressure areas over the relatively warm North Atlantic and North pacific Oceans (Iceland low and Aleutian low)
In the southern hemisphere, little change occurs from summer to winter due to global encirclement by the southern oceans.
4. Polar high pressure belts (Polar highs)
On the average, high pressure areas exist over both polar regions, but intensities and locations of the centers are known to shift, being only rarely entered on the geographic poles.
B. PLANETARY WIND SYSTEMS
B-1. Surface Winds (winds in the lower troposphere)
1. Tradewinds