From the point of view of air masses, the largest continent of Asia is a land of extremes. Since the landmass of this continent extends from near about the North Pole to the Equator, the hottest as well as the coldest air masses are found here.
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The wintertime and the summertime continental as well as maritime air masses exhibit greater extremes of temperature than is found in any of the other continents. However, in certain respects the air masses of Asia and North America are similar.
But in the absence of upper air data, detailed information about Asian air masses is still lacking. The principal air masses of this continent, which undergo considerable modifications according to the changing seasons, have been described in brief in the following paragraphs.
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(1) Continental polar (cP) air masses:
During the winter season the continental polar air masses originate in the cold source regions of central-eastern Siberia and Outer Mongolia. They are very cold and dry in their source regions. They may be compared with their counterparts in North America which originate over the snow and ice covered areas of north Canada.
A temperature of -40° Celsius is recorded in the lower portion of the continental polar air masses in Siberia. The temperature and humidity of the north Siberian air masses are much lower than those of the south Siberian air masses.
There is an inversion layer found invariably in the cold and comparatively drier air masses. When the air masses move towards the Pacific, the mountain barriers produce mechanical turbulence in them and the inversion layer is broken.
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Far away from the source regions, the temperature and the humidity in the lower portion of air masses register a marked increase. During the winter months the anticyclones produced in central Asia make the influence of these polar continental air masses felt over a very large part of the continent. The high plateaus of Tibet and the lofty ranges of the Himalayas do not allow the air masses to invade the Indian sub-continent.
The above air masses enter into China by land and sea both. Those entering China by land have higher temperatures-about 11°-19°C more than the source regions. When the modified cP air masses enter the Indian sub-continent from the western end of the mountain barrier, they produce winter precipitation in association with the extra-tropical cyclones.
The northern and western parts of the sub-continent receive moderate to scanty rainfall. Modified cP air mass also enters the North Vietnam from the eastern extremities of the highlands.
During the summer months, eastern and southern Asia is under the influence of summer monsoon, hence cP air masses are rather unimportant in the above region.
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But Japan and the extreme north of China experience the continental polar air masses to a certain degree in the summer season. Wherever the cPs air masses pass over the cool ocean surface, they become more humid in the lower parts.
(2) Maritime polar (mP) air masses:
In winter, because of the westerly circulation in eastern Asia, the coastal regions south of Korea remain outside the influence of mP air masses. Their influence is limited to the coastal regions of Siberia, Manchuria and Korea.
The mainland of China remains completely free from them. The mP air masses that originate over the Sea of Okhotsk control the weather of the east coast regions in summer. Manchuria and eastern Siberia are also under their control in the summer months.
According to the Japanese meteorologists, in the early summer the maritime polar air masses from the Sea of Okhotsk invade Japan and sometimes form a front with the overlying mT air masses. The weather along this front is generally cyclonic with overcast sky and light precipitation. Such weather is called Bai-u in Japan.
(3) Maritime tropical (mT) air masses:
In summer, entire Southeast Asia is dominated by summer monsoon. In fact, the air masses involved in the summer monsoon circulation are maritime tropical. They are hot and humid with convective instability in them. Even a slight uplift of these air masses causes torrential rainfall.
When the mT air masses move into continental land areas, they are heated from below which produces convectional currents in them. Whenever they are forced to ascend a mountain barrier or are dragged into a cyclonic circulation, heavy summertime precipitation is the result.
When the equatorial southwesterly air currents continue up to North China and Manchuria, they encounter the cP air masses and the result is the generation of polar fronts. At the polar front, the uplift of hot and moist air produces heavy summer precipitation.
The weather of Japan and Korea is also affected by these air currents. Another current of the summer monsoon originates over the western margin of the Pacific anticyclone which reaches Japan and eastern China from a southern, southwestern or southeastern direction.
In the winter, the maritime tropical air masses originate over the southern portions of the oceans. In view of the vast size of the continent and topographical features, mT invasions are rare. However, maritime air masses with marked instability are seen in the southwest Pacific.
They are not only convectively unstable, but have a very high moisture content as well as high surface temperatures. Eastern Indonesia is dominated by the unstable tropical maritime air masses originating from the cP air masses.
Winter cyclones moving northward from the southwest Pacific are accompanied by mT air masses. Since the winter cyclones generally follow a maritime trajectory, the winter mT air masses are seldom seen over the land.
During the periods of intense cyclonic activity, Eastern Europe is invaded by the maritime tropical air masses only at upper levels. They originate from the eastern side of the subtropical Atlantic anticyclone.
They are, therefore, characterized by moderately high temperatures and are convectively unstable. Sometimes Southwest Asia is invaded by surface air masses which resemble the mild mT air mass. They move across Turkey ahead of the eastward- moving winter cyclones.
These air masses pick up large quantities of moisture from over the eastern Mediterranean Sea. Their temperature is also fairly high. Similarly, when the wintertime cyclones move across the Persian Gulf and northern India, they are often associated with warm mT air masses.
These air masses differ from typical maritime tropical air masses as they are only modified or returning continental polar air masses.
South Asia during the winter Season is dominated by the northeast trades which have maritime tropical characteristics in the coastal regions, but develop cT characteristics when they move over the more extensive areas of the Southeastern and Indian Peninsulas.