Following factors affect the properties of the air-mass:
- Its Source region: if it is polar origin, the air-mass will be cold and if of tropical origin, warm.
- Its track over the earth’s surface: if it passes over large expanses of water, it will be moist and if it passes over land, it will be dry.
- The extent of convergence and divergence: convergence at lower levels (as in depressions) causes air at the centre to ascend. Divergence at lower levels ( as in anticyclones) causes air from above to descend. Both convergence and divergence causes vertical movement of air resulting in the proper mixing of the air of different levels and thus they influence the properties of the air-mass, especially relative humidity and temperature lapse rates.
- Its age: the age of an air-mass is the number of days it has spent in its source region. The longer it has stayed there, the greater it is influenced by the climate of that place. For a tropical air-mass to get converted into polar air-mass or vice-versa, it may take well over a month.
- Its rate of travel: consider an air-mass moving over an area outside its source region. If it moves quickly, the area over which it blows doesn’t have sufficient time to significantly influence the properties of the air-mass. Hence a quick moving airmass retains most of its original characteristics. On the other hand if the airmass moves slowly, the area over which it blows has sufficient time to influence the properties of the air-mass. Hence the characteristics of a slow moving air-mass may be different from its original character.
