Air Masses and Fronts

Air Masses and Fronts

The study of air mass is very important part of Meteorology. Air always takes some of the properties of the area over which it lies. This parcel of air may remain stationary for several days and develops its own characteristics. Under this situation, the air becomes recognisable as an air mass.

An air mass is defined as ‘an immense body of air several kilometres in length and breadth and thickness which is characterised by homogeneous physical properties (like temperature, moisture) in horizontal direction at any level’.

Such an extensive portion of the surface area over which air mass has acquired its qualities is called as ‘Air mass source region’. The source region may be land or water body. For example, Sahara desert, Siberia, the Great Plain of North America, Northern Plain of Europe, Western Australia, Antarctica, Green Land, Arctic Ocean, Northern and Southern Pacific, Atlantic Oceans are favourable locations as source region for air masses.

The air masses can be classified based on the following factors;

a.        Latitude - Tropical(T) and Polar (P) air masses

b.        Nature of the surface – Continent (c) and marine (m) air masses

c.         Temperature – warm (w) and Cold (k) air masses

d.        Stability – stable (s) and unstable (u) air masses

Air masses normally migrate from their source region to other regions, which have different surface properties, mostly along with primary winds. As the air masses move out from their source regions, they not only modify the weather of the areas they occupy, but also modify themselves according to the surface over which it moves.

Fronts

When two air masses with different physical characters meet, there is usually little mixing of the air. 

The zone of transition between two contrasting air masses is called as ‘Fronts’. As heavier air mass always tends to push up the lighter air mass, the front always slopes over the cold air mass.

If cold air mass moves forward rapidly and causes the warm air to rise vertically, it leads to the formation of steep slope called as ‘Cold front’. Cold fronts result in the formation of cumulo nimbus clouds with heavy rainfall associated with lightning and thunder.

If warm air is moving over cold air mass, it produces a gentle slope called as ‘Warm front’. Warm fronts result in the formation of stratus and nimbo stratus clouds and cover over large areas, leading to moderate rainfall (Figure 6.26).