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What is an Air Mass?
An air mass is an immense body of air moving over the earth’s surface as a recognisable entity, having uniform physical properties at a given altitude.
Air masses form an integral part of the global planetary wind system. Therefore, they are associated with one or other wind belt.
Cold Air Mass:
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A cold air mass is one which is colder than the underlying surface and is associated with instability and atmospheric turbulence.
Warm Air Mass:
A warm air mass is one which is warmer than the underlying surface and is associated with stable weather conditions.
Source Region:
Of an air mass is one which establishes heat and moisture equilibrium with the overlying air mass due to prolonged contact. The nature of source region determines the temperature and humidity characteristics of an air mass. An ideal source region should be extensive, broadly uniform with gentle, divergent air circulation. Areas with high barometric pressure and low barometric gradients are ideal source regions. There are no major source regions in the mid-latitudes as these are dominated by cyclonic and other disturbances.
When an air mass moves away from a source region, the upper level maintains the physical characteristics for a longer period, because convection is not possible in stable, stagnant air, conduction is slow, and radiation is not effective in free air.
Influence of Air Masses on World Weather:
The properties of an air mass which influence the accompanying weather are vertical distribution temperature (indicating its stability and coldness or warmness) and the moisture content.
The moving air masses undergo the following modifications:
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Thermodynamic which depends on the initial temperature, moisture conditions, path taken, underlying area and the time taken?
Mechanical by eddies, convection in lapse- rate, subsidence, ascent, lateral spreading, divergence and convergence.
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The air masses carry atmospheric moisture from oceans to continents and cause precipitation over landmasses. They transport latent heat, thus removing the latitudinal heat balance. Most of the migratory atmospheric disturbances such as cyclones and storms originate at the contact zone between different air masses and the weather associated with these disturbances is determined by characteristics of the air masses involved.
Classification of Air Masses:
Broadly, the air masses are classified into polar and tropical air masses.
Both the polar and the continental air masses can be either of maritime or continental types. The four types of air masses with their source- regions are shown in Fig. 2.19, and their characteristic features are discussed below.
1. Continental Polar Air Masses (CP):
Source- regions of these air masses are the Arctic basin, northern North America, Eurasia and Antarctica. These air masses are characterised by dry, cold and stable conditions with very cold and dense air during winter at the source-region due to prolonged terrestrial radiation and distance from warm oceans.
The wind circulation is gentle and divergent. The weather during winter is frigid, clear and stable with meager cloudiness and precipitation. During summer, the weather is less stable with lesser prevalence of anticyclonic winds, warmer landmasses and lesser snow. (Fig. 2.19).
2. Maritime Polar Air Masses (MP):
The source-region of these air masses are the oceans between 40″ and 60° latitudes. These are actually those continental polar air masses which have moved over the warmer oceans, got heated up and have collected moisture. The conditions over the source-regions are cool, moist and unstable. These are the regions which cannot lie stagnant for long. The weather during winters is characterised by mild temperatures, high humidity, overcast skies and occasional fog and precipitation. During summer, the weather is clear, fair and stable.
3. Continental Tropical Air Masses (CT):
The source-regions of these air masses include tropical and sub-tropical deserts of Sahara in Africa, and of West Asia and Australia. These air masses are dry, hot and stable at source-region and do not extend beyond the source. They are thus localised. They are dry during both summer and winter and very hot during summer with scanty rainfall throughout.
4. Maritime Tropical Air Masses (MT):
The source-regions of these air masses include the oceans in tropics and sub-tropics such as Mexican Gulf, the Pacific and the Atlantic oceans. These air masses are warm, humid and unstable over the source-regions. Instability is prominent over the western margins of oceans. The weather during winter has mild temperatures, overcast skies with fog and occasional drizzle. During summer, the weather is characterized by high temperatures, high humidity, cumulous clouds and convectional rainfall.