QUES . What are the important factors responsible for airmass modifications?
HINTS:
Modification or transformation of air mass occurs when air masses move from their source region to another region. Where the air mass moves, there is an exchange of heat, cold, and moisture with the underlying surface.
In what ways (processes) air masses are modified?
Air masses are modified in several ways. For the most part, these are processes which we have already considered in detail. Several of the processes usually take place concurrently:
1 . An air mass is heated from below if it passes over a warmer surface (previously warmed by the sun) or if the surface beneath a slow-moving air mass is being currently warmed by the sun. Such modification is rapid because of the resulting instability and convection.
2 . An air mass is cooled from below if it passes over a colder surface, or if the surface is cooled by radiation. This increases the stability of the lower layers, and further modification becomes a slow process.
3 . Moisture may be added to an air mass by: (a) Evaporation from water surfaces, moist ground, and falling rain; (b) sublimation from ice or snow surfaces and falling snow or hail; and (c) transpiration from vegetation. Of these, sublimation is a relatively slow process by comparison.
4 . Moisture may be removed from an air mass by condensation and precipitation.
5 . Finally, air-mass properties may be changed by turbulent mixing, by sinking, or by lifting.
After moving a considerable distance from its source region, particularly after entering a source region of another type, an air mass may lose its original distinctive characteristics entirely and acquire those of another air-mass type. Thus, a continental polar air mass moving out over the Gulf of Mexico takes on the characteristics of a maritime tropical air mass. Or a maritime polar air mass, after crossing the Rocky Mountains, may assume the characteristics of a continental polar air mass.
If the air mass moves over a surface that is warmer (such as continental polar air moving out from the continent in winter over warmer water), the warm water heats the lower layers of the air mass, increasing instability (decreasing in stability), and consequently spreading to higher layers.
Factors responsible for the modification of air mass
1 . Nature of the underlying surface. 2 . Stability or instability of air masses. 3 . The apparent movement of the Sun.
1 . Nature of the underlying surface:
Underlying surfaces are the main deciding factors of the properties of air masses. The air mass which is above the tropical ocean are moist and warm. The air mass above the continent and higher latitudes are generally dry and cold.
If the destination region is warmer than the air mass, then the temperature of the air masses increases. In this way, the cold air mass gets converted into warm air mass.
If the destination region is colder than the air mass, then the temperature of the air mass decreases, in this way the warm air mass gets converted into the cold air mass.
If the air mass moves from the continent to the ocean, then its moisture increases, and on the contrary, if the air mass moves from the ocean to the continent, then its moisture decreases.
2 . Stability or instability of air masses
Stable air masses (such as continental polar air mass ) do not move and thus they do not modify easily.
Unstable air masses such as tropical maritime air mass move frequently and they are modified.
3 . The apparent movement of the Sun
When the Sun moves from the Southern Hemisphere to the Northern Hemisphere, all the Southern Hemisphere air masses also follow the Sun’s motion and move to the North. The same happens when the Sun moves from the Northern Hemisphere to the Southern Hemisphere.
For example, In the summer season, tropical maritime air masses of the Atlantic ocean move over the Indian subcontinent. In the winter season, the Continental polar airmass of Alaska and Canada move southern ward and came over the middle of the North American continent.
