In this article we will discuss about the wintertime and summertime air masses of Asia.
1. Wintertime Air Masses:
i. Continental Polar Air Masses:
These air masses originate over extensive areas comprising Siberia and outer Mongolia having very cold ground surface. Initially, the air masses are very cold and dry in their source regions. The lower portion upto the height of one kilometre is characterized by inversion of temperature.
The air masses move eastward and after covering long distances are mechanically modified as mechanical turbulence is produced when these air masses cross over the mountain barriers. This process leads to the disappearance of inversion layer resulting into increase of temperature and humidity in the lower layer.
These air masses enter China through two routes viz.:
(i) Through land surface, and
(ii) Through water surface.
When high pressure lies over Mongolia and North China, then these air masses enter China by land route. They are much warmer in China than in their source areas. These air masses are associated with clear sky and dry weather and cold air.
When these air masses come with high velocity, they bring with them immense quantity of dust and sands and deposit them as loess. The continental polar air masses in their modified forms affect the weather conditions of most parts of Asia during winter season. These air masses do not enter the Indian subcontinent because of effective barrier of the Himalayas.
When high pressure lies over Manchuria and Japan Sea, the continental polar air masses enter China by sea route after moving over Japan Sea, and Yellow Sea and thus pick up abundant moisture. These air masses are relatively warmer and more humid than the continental polar air masses coming by land route.
Until they are associated with fronts, they are characterized by clear sky and pleasant weather. The lower portion is unstable and thus they give precipitation when they ascend along the mountain barriers. The continental air masses coming through sea and land routes converge along the eastern coasts of Asia and form cyclones through frontogenesis and cause precipitation.
ii. Maritime Polar Air Masses (mP):
These air masses after originating over the Sea of Okhotsk influence only the coastal margins of Siberia, Manchuria and South Korea while the eastern coasts of Asia south of Korea are deprived of their influences because (i) the winter air circulation is off shore i.e., from west to east due to which the westward advance of maritime polar air masses is blocked, and (ii) continental polar air mass while entering China through sea route attains the characteristics of maritime polar air mass (mP). These air masses (mP) also invade Japan in early summer and form fronts when they converge with overlying maritime tropical air masses and bring moist weather with overcast sky and light precipitation.
iii. Maritime Tropical Air Masses (mT):
Maritime tropical air masses do not effectively influence the weather conditions of the eastern Asia during winters because of the dominance of continental polar air masses. These air masses are experienced only up to southern China. They are warmer and more humid than all of the wintertime air masses. Unstable maritime tropical air masses are more effective in south-west Pacific Ocean and in eastern Indonesia.
2. Summertime Air Masses:
i. Continental Polar Air Masses (cP):
The source areas of polar air masses extend further northward in central Asia because of high temperature during summer season due to northward migration of the sun. The air becomes relatively warm but continental polar air masses do not effectively influence the weather conditions of eastern and southern Asia because maritime tropical air masses become more dominant during summer season.
The continental polar air masses enter China only through sea route from Japan Sea and Yellow Sea. These air masses are colder than maritime tropical air masses. They are associated with clear weather, scant precipitation, and negligible thunderstorms. They produce cyclonic conditions whenever they converge with maritime tropical air masses.
ii. Maritime Tropical Air Masses (mT):
The weather of south and south-east Asia is controlled by maritime tropical air masses which are known as summer monsoons. They are warm, more humid and unstable. They yield torrential rainfall when they are forced to ascend by mountain barriers largely. After being originated in southern oceans they move north and north-eastward and after entering the mainland they are heated from below because of warm ground surface and hence they become unstable and convectional currents are produced.
The south-west summer monsoons of Indian subcontinent are typical representatives of maritime tropical summer air masses. These air masses produce cyclonic conditions when they converge with continental polar air masses during springs in central China and during middle summer in Manchuria.
iii. Maritime Polar Air Masses (mP):
These air masses originate over Okhotsk Sea from where they move westward and influence the weather conditions of eastern Asia north of 40°N latitude. These air masses are more active during summers than during winters. They are more effective in Manchuria and east Siberia. Though these air masses extend up to southern Japan in early summer but later they are pushed northward by maritime tropical air masses.