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DYNAMICAL (ATMOSPHERIC CIRCULATION) CONTRASTS

All dynamical differences between the hemi-spheres stem from one fact; coriolis force deflects to the left (negative coriolis parameter) in the Southern Hemisphere. For example, the basic relationship between wind and pressure, as expressed in Buys Ballots law, changes in the Southern Hemisphere: With your back to the wind, low pressure is always to your right. Consequently, the sense of rotation about lows is clockwise and about highs is counterclockwise in this hemisphere. In either hemisphere, how-ever, lows are still called cyclones and highs, anticyclones.

MEAN PRESSURE CHARACTERISTICS

The mean pressure distribution over the Southern Hemisphere shows three large semiper-manent highs at 30S over the oceans. (See fig. 3-2-1.) One is in the eastern South Pacific, extending from about 140W to the west coast of South America; a second almost completely covers the South Atlantic Ocean; and the third one more or less centrally placed over the South Indian Ocean. South of this high-pressure belt, pressure decrease is regu-lar and marked to about 65S where a con-tinuous low-pressure trough, known as the "Antarctic Trough," encircles the Antarctic. Farther south is found higher pressure, over Antarctica. From the equator to 30S is a region of low pressure. In the summer, heat lows appear south of the Amazon basin in South America, over the whole of the eastern part of South Africa, and over northern Australia. The latter is the largest of these heat lows and covers northern Australia, the Dutch East Indies, and the central Indian Ocean. In the winter, although pressure is relatively high over land, separate subtropical anticyclones can still be distinguished. The only pronounced seasonal variation of pressure in mid-latitudes is the shift of the axes of the subtropical highs which move only a few degrees of latitude northward during the Northern Hemisphere sum-mer and few degrees of latitude southward in the Northern Hemisphere winter.

AIR MASSES

The semipermanent highs produce tropical air similar to the highs in the Northern Hemisphere. There is no continental polar air in the Southern Hemisphere. The air over the snow- and ice-covered regions is antarctic air, but it rarely leaves this area as true antarctic air. It becomes rapidly modified to maritime polar air as it moves over the water. Maritime polar air is the most predominant air of the Southern Hemisphere. Air forming in the central dry regions of South Africa and Australia has all the properties of continental tropical air and, in its source region, is dry and cloudless with a steep lapse rate. When it moves to neigh-boring oceans, a strong inversion is formed in the lower layers, and with the addition of moisture from the ocean, sheets of strato-cumulus and stratus are formed.

MAJOR FRONTAL ZONES

In figures 4-2-6 and 4-2-7 you can see the major frontal zones of the Southern Hemisphere. Areas of frontogenesis are found in the semi-stationary polar troughs that exist along the western border of each of the subtropical highs. Wave disturbances form along these fronts much the same as those forming along the polar fronts in the Northern Hemisphere. Usually they develop into families of from two to six. This wave sequence ends when the final wave member of the series has run together and occluded in a large central cyclone to the southeast of the semipermanent highs.

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