General Atmospheric Circulation
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 ND develops a low relative humidity and clear skies dryness of descending alarm Is responsible for the prevalence of world’s greatest deserts either in or adjacent to the horse latitudes. N 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. Subpoena 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. On the average, high pressure areas exist over both polar regions, but intensities and actions of the
B. PLANETARY WIND SYSTEMS B-l . Surface Winds (winds in the lower troposphere) 1 . Transcends (Tropical easterlies) winds blowing from the horse latitudes to the doldrums best developed over the Atlantic and much broader Pacific oceans, away from the pressure perturbations of the continents tend to follow the pressure gradient directed toward the equator, but become deflected to the right (in the north hemisphere) and to the left (in the south mesosphere) becoming the Northeast Transcends and the Southeast Transcends, respectively. Eave an average speed of 10-16 knots and strongest over the North hemisphere oceans during summer when the subtropical high is strongest the largest in area of the planetary wind groups due to the diminishing area within latitudinal zones from equator to poles 2. Prevailing Westerly winds blowing from the horse latitudes to the stormy latitudes; southwesterly and northwesterly due to acropolis deflection in the north and south hemisphere, In the southern hemisphere, a relatively strong and uniform pressure gradient is present between the horse latitudes and the stormy latitudes.
Associated westerly are quite steady and strong throughout the year with an average force of 17-27 knots, progressive zone being designated as “roaring forties, furious fifties and screaming sixties”. In the northern hemisphere, prevailing westerly are variable and often masked by prominent circulation about moving low and high pressure areas or completely reversed as reversals in seasonal pressure of the related horse latitude and subpoena sows occur. . Monsoon winds winds whose direction reverses with the season, due to a reversal in the pressure reversal of pressure gradient results from changes in land temperature from winter to summer most notable and widespread pressure change is the Asiatic monsoon circulation and that of west and southwest of African bulge in the Atlantic region arise from temperature differentials between large water and land bodies 8-2. Winds in the upper atmosphere.