Quantcast Air masses in the southern hemisphere

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AIR MASSES IN THE SOUTHERN HEMISPHERE

The air masses of the Southern Hemisphere are predominantly maritime. This is because of the overwhelming preponderance of ocean areas. Great meridional (south-north and north-south) transports of air masses, as they are known in the Northern Hemisphere, are absent because the westerlies are much more developed in the Southern Hemisphere than in the Northern Hemisphere. Except for Antarctica, there are no large land masses in the high latitudes in the Southern Hemisphere; this prevents sizable inva-sions of antarctic air masses. The large land masses near the equator, on the other hand, per-mit the extensive development of warm air masses.

The maritime tropical air masses of the Southern Hemisphere are quite similar to their counterparts of the Northern Hemisphere. In the large area of Brazil, there are two air masses for consideration. One is the regular air mass from the Atlantic which is composed of unmodified mT air. The other originates in the Atlantic; but by the time it spreads over the huge Amazon River basin, it undergoes two important changes—the addition of heat and moisture. As a result of strong summer heating, a warm, dry continental tropical (cT) air mass is located from 30° south to 40° south.

The maritime polar air that invades South America is quite similar to its counterpart in the United States. Maritime polar air occupies by far the most territory in the Southern Hemisphere, encircling it entirely.

Australia is a source region for continental tropical air. It originates over the vast desert area in the interior. Except along the eastern coast, maritime tropical air does not invade Australia to a marked degree. This air is brought down from the north, particularly in the summer, by the counterclockwise circulation around the South Pacific high. 

Antarctica is a great source region for intensely cold air masses. The temperatures are colder than in the arctic regions. These air masses have continental characteristics, but before the air reaches other land areas, it becomes modified and is properly called maritime polar.

During the polar night the absence of insola-tion causes a prolonged cooling of the snow sur-face, which makes Antarctica a permanent source of very cold air. It is extremely dry and stable aloft. This polar air mass is referred to as continental antarctic (cA) air.

In summer the continent is not as cold as in winter because of constant solar radiation but continues to function as a source for cold cA air. In both winter and summer, the air mass is thermally modified as it flows northward through downslope motion and surface heating; as a result, it becomes less stable. It assumes the characteristics of maritime antarctic air. The leading edge of this air mass then becomes the northern boundary of the antarctic front. To the north of the antarctic front is found a vast mass of maritime polar air that extends around the hemisphere between 40°S and 68°S in summer and between 34°S and 65°S in winter. At the northern limit of this air mass is found the Southern Hemisphere polar front. During sum-mer this mP air is by far the most important cold air mass of the hemisphere because of the lack of massive outbreaks of cold continental air from Antarctica.

Different weather conditions occur with each type of air mass. The cA air produces mostly clear skies. The mA air masses are characterized generally by an extensive overcast of stratus and stratocumulus clouds with copious snow showers within the broad zone of the antarctic front. An area of transition that extends mainly from the coastline to the northern edge of the consolidated pack ice is characterized by broken to overcast stratocumulus clouds with somewhat higher bases and little precipitation.

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