Formation and Modification of Air Masses

Air Mass Formation

The region where an air mass acquires its characteristic properties of temperature and moisture is called its source region. Ocean areas, snow- or ice-covered land areas, and wide areas are common source regions. Those areas producing air masses which enter the fire-occurrence regions of North America are:

1. The tropical Atlantic, Caribbean, Gulf Mexico and the tropical Pacific, which are uniformly warm and moist.
2. The Northern Pacific and Northern Atlantic, which are uniformly cool and moist.
3. Interior Alaska, Northern Canada, and the Arctic, which are uniformly cold and dry during the winter months.
4. Northern Mexico and Southwestern States, which are usually hot and dry during the summer months.

The time required for a body of air to come to approximate equilibrium with the surface over which it is resting may vary from a few days to 10 days or 2 weeks, depending largely on whether the body of air is initially colder or warmer than the temperature of its source region. If the air is colder, it is heated from below. Convective currents are produced, which carry the heat and moisture aloft and rapidly modify the air to a considerable height.

On the other hand, if the air is initially warmer than the surface, it is cooled from below. This cooling stabilizes the air and cuts off convection. Cooling of the air above the surface must take place by conduction and radiation, and these are slow processes. Thus, a longer time--up to 2 weeks--is required for the development of cold air masses, and even then, these air masses are only a few thousand feet thick.

Air masses that form over a source region vary in temperature and moisture from season to season, as does the source region. This is particularly true of continental source regions. High-latitude continental source regions are much colder and drier in the winter than in the summer, and tropical continental source regions are much hotter and drier in summer than in winter.

Classification of Air Masses

Air masses are classified according to their source region. Several systems of classification have been proposed, but we will consider only the simplest. Air masses originating in high latitudes are called polar (P), and those originating in tropical regions are called tropical (T). Air masses are further classified according to the underlying surface in the source region as maritime for water and continental for land. The "m" for maritime or "c" for continental precedes the P or T. Thus, the four basic types of air masses, are designated as: mP, mT, cP, and cT, according to their source region. It is natural that air stagnating for some time in a polar region will become cold, or in a tropical region will become warm. And air spending sometime over water becomes moist, at least in the lower layers, while air over land becomes dry.

For convenience, the four basic air mass types are often referred to as moist cold, moist warm, dry cold, and dry warm.

Air Mass Modification

As an air mass leaves its source region in response to broadscale atmospheric motions, it may be colder or warmer than the surface it passes over. It is then further classified by the addition of k for colder or w for warmer to its classification symbol. The k-type air mass will be warmed from below and will become unstable in the lower layers. A w-type air mass will be cooled from below, will become stable, and will be modified slowly, and only in the lower few thousand feet.

Air-mass properties begin changing as soon as the air mass leaves its source region. The amount of modification depends upon the speed with which the air mass travels, the type of surface over which it moves, and the temperature difference between the air mass and the underlying surface.

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:

• 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.
• 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.
• Moisture may be added to an air mass by:
  • Evaporation from water surfaces, moist ground, and falling rain;
  • sublimation from ice or snow surfaces and falling snow or hail; and
  • transpiration from vegetation. Of these, sublimation is a relatively slow process by comparison.
• Moisture may be removed from an air mass by condensation and precipitation.
• 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.