Airmasses
An airmass is a large body of air with relatively uniform thermal and moisture characteristics. Airmasses cover large regions of the earth, typically several hundred thousand square kilometers. Airmasses can be as deep as the depth of the troposphere or as shallow as 1 to 2 km.
Airmasses form when air remains over a relatively flat region of the earth* with homogeneous surface characteristics for an extended period of time. ( Canadian and Siberian plains, cool oceanic regions such as the North Atlantic and Pacific, deserts, such as the Sahara and the American southwest, and tropical oceanic regions including the equatorial Atlantic and Pacific, and smaller water bodies such as the Caribbean Sea and the Gulf of Mexico).
Polar air masses, containing little moisture and low temperatures move downward from the poles. Air masses that form over water are generally moist, and those that form over the tropical oceans are both moist and warm. Because of the Coriolis effect due to the Earth’s rotation, air masses generally move across North America from west to east. But, because of the differences in moisture and heat, the collision of these air masses can cause instability in the atmosphere.
Polar air mass is cold and tropical air mass is warm. When cold air mass and warm air mass blow against each other, the boundary line of convergence separating the two air masses is termed as front. When the warm air mass, moves upward over the cold air mass the front formed in such a situation is called warm front. On the contrary, when the cold air mass advances faster and undercuts the warm air mass and forces the warm air upwards, the front so formed is called cold front. The frontal surface of cold front is steeper than that of a warm front . A prevailing air mass in any region – polar, tropical, maritime or continental largely controls the regions general weather.
Different air masses are:-
- Maritime tropical (mT)
ii. Continental tropical (cT)
iii. Maritime polar (mP)
iv. Continental polar (cP)
v. Continental arctic (cA).
Where ‘m’ stands for Maritime; ‘c’ stands for continental; ‘T’ stands for tropical; ‘P’ stands for polar and ‘A’ stands for arctic region.
Fronts
An important properties of air is that it is a poor conductor of energy. This means that when two different bodies of air come together, they do not readily mix. Rather, each body of air will retain its individual properties, and a boundary forms between them. When two large air masses meet, the boundary that separates them is called a front. Fronts represent fairly abrupt transitions between two large air masses. The warm, moist air might dominate an area hundreds of miles across,
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