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.
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, while in another part of the continent a cold, dry air mass holds sway over an equally large region. However, where the two air masses meet, the transition layer between them may be only a few tens of miles across, clearly a sharp transition between two massive bodies of air.
Fronts are recognized by the following properties:-
- Sharp temperature changes over a relatively short distance. Sometimes change of 10 to 20 C may be observed.
- Change in moisture content
- Rapid shifts in wind direction
- Pressure changes
- Clouds and precipitation patterns
Types of Fronts:-
Warm Fronts: A warm front occurs when a warm air mass advances and replaces a cold air mass. On a weather map, a warm front is depicted as a red arc, with red semicircles pointing in the direction of the advancing warm air.
Cold Fronts :-A cold front occurs when a mass of cold air advances into a region of warmer air.
Stationary Fronts:- A stationary front forms when a cold front or warm front stops moving. This happens when two masses of air are pushing against each other but neither is powerful enough to move the other. Winds blowing parallel to the front instead of perpendicular can help it stay in place.
Occluded Fronts:- Sometimes a cold front follows right behind a warm front. A warm air mass pushes into a colder air mass (the warm front) and then another cold air mass pushes into the warm air mass (the cold front). Because cold fronts move faster, the cold front is likely to overtake the warm front. This is known as an occluded front