Air masses normally migrate from their source regions to other regions which have different surface properties. As the air masses move out from their source regions, they not only modify the weather of the areas they occupy, but they are also modified to a certain extent by the surface over which they are moving.
As air masses move away from their source regions, they retain all the physical properties. These initial conditions of an air mass, as they are referred to, include its vertical temperature distribution, moisture content and its distribution, mean temperature, velocity, and wind velocity, etc. Variation in these physical characteristics is found in the vertical distribution.
Air masses have the unique property of being homogenous in their horizontal extent. But the climatic characteristics of the areas traversed by air masses do modify them.
Warming or cooling from below, changes in radiation rates, addition of moisture or loss of it, subsidence or uplift are some of the factors which bring about changes in an air mass.
Thus, it is clear that the weather of a region dominated by a particular air mass depends to a large extent on the changes that have been brought about in the air mass itself.
There are two principal types of air mass modifications: (a) thermodynamic modifications, and (b) mechanical modifications. The first are thermodynamic in origin, and the second arise from mechanical causes. These modifications may occur either separately or in combination.