Maritime Tropical (mT) Air in WinterMaritime tropical air that arrives over Europeusually originates over the southern portion of theNorth Atlantic under the influence of the Azoresanticyclone. Maritime tropical air is marked bypronounced stability in the lower layers and typicalwarm-mass cloud and weather conditions. Relativelyhigh temperatures accompany the influx of mT air, andthe moisture content is greater than in any other airmass observed in the middle latitudes of Europe.Visibility is, as a rule, reduced because of the presenceof fog and drizzle, which are frequently observed withan influx of mT air. Maritime tropical air in winterexists only in western Europe. By the time it reachesRussia, it is generally found aloft and greatly modified.Maritime Tropical (mT) Air in SummerIn general, mT air has the same properties as itscounterpart in winter with the exception that it is lessstable over land because of surface heating.Additionally, this air mass loses its maritimecharacteristics soon after passing inland.Over water, mT air is still a typical warm air mass.Sea fog frequently occurs in the approaches to theEnglish Channel during the spring and early summer.Visibility in mT air is generally better in summer thanin winter, particularly over land where convectioncurrents usually develop.MaritimetropicalairflowingovertheMediterranean in summer usually changes to a coldmass, since the water temperature of the Mediterraneanis then slightly higher than that of the air. Weakconvection currents prevail, usually sufficiently strongto form cumulus clouds but seldom sufficiently strongto produce showers.Continental Tropical (cT) Air in WinterThe continental tropical air that arrives over Europein winter originates over North Africa. By the time itreaches central Europe, it differs little from mT air. Ingeneral, a cT air mass is much more prevalent oversouthern Europe than over central or western Europe.Although the moisture content of cT air is less than thatobserved in mT air, the visibility is not much better,primarily because of the dust that cT air picks up whileover Africa. This air mass constitutes the major sourceof heat for the development of the Mediterraneancyclonic storms, most common during the winter andspring months.Continental Tropical (cT) Air in SummerThe cT air usually develops over North Africa, AsiaMinor, and the southern Balkans. At its source region,the air is dry and warm as well as unstable. The NorthAfrican air mass is the hottest air mass on record in theworld. In its northward flow over southern Europe, cTair absorbs moisture and increases its convectiveinstability. The summer showers and thunderstormsobserved over southern Europe are often produced by amodified cT air mass. This air mass is much moreprevalent over southern Europe than is its wintercounterpart.AIR MASSES IN THE SOUTHERNHEMISPHEREThe air masses of the Southern Hemisphere arepredominantly maritime. This is because of theoverwhelming preponderance of ocean areas. Greatmeridional (south-north and north-south) transports ofair masses, as they are known in the NorthernHemisphere, are absent because the westerlies aremuch more developed in the Southern Hemisphere thanin the Northern Hemisphere. Except for Antarctica,there are no large landmasses in the high latitudes in theSouthern Hemisphere; this prevents sizable invasionsof antarctic air masses. The large landmasses near theequator, on the other hand, permit the extensivedevelopment of warm air masses.The maritime tropical air masses of the SouthernHemisphere are quite similar to their counterparts ofthe Northern Hemisphere. In the large area of Brazil,there are two air masses for consideration. One is theregular air mass from the Atlantic, which is composedof unmodified mT air. The other originates in theAtlantic; but by the time it spreads over the hugeAmazon River basin, it undergoes two importantchanges—the addition of heat and moisture. As a resultof strong summer heating, a warm, dry continentaltropical (cT) air mass is located from 30° south to 40°south.The maritime polar air that invades South Americais quite similar to its counterpart in the United States.Maritime polar air occupies by far the most territory inthe Southern Hemisphere, encircling it entirely.Australia is a source region for continental tropicalair. It originates over the vast desert area in the interior.Except along the eastern coast, maritime tropical airdoes not invade Australia to a marked degree. This air isbrought down from the north, particularly in the4-19
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