The effect of topography is evident primarily in the mountainous regions. The air mass is modified on the windward side by the removal of moisture through precipitation with a decrease in stability; and, as the air descends on the other side of the mountain, the stability increases as the air becomes warmer and drier.

After an air mass has left its source region, the trajectory it follows (whether cyclonic or an-ticyclonic) has a great effect on its stability. If the air follows a cyclonic trajectory, its stability in the upper levels is decreased; this instability is a reflection of cyclonic relative vorticity. The sta-bility of the lower layers is not greatly affected by this process. On the other hand, if the trajec-tory is anticyclonic, its stability in the upper levels is increased as a result of subsidence associated with anticyclonic relative vorticity.

Although the age of an air mass in itself can-not modify the air mass, it does determine (to a great extent) the amount of modification that takes place. For example, an air mass that has recently moved from its source region cannot have had time to become modified significantly. However, an air mass that has moved into a new region and stagnated for some time is now old and has lost many of its original characteristics.

The stability of an air mass often determines the type of clouds and weather associated with that air mass. The stability of an air mass can be changed by either thermodynamic or mechanical means.

THERMODYNAMIC. The thermodynamic influences are reflected in a loss or gain in heat and in the addition or removal of moisture. 

Heat Loss or Gain. The air mass may lose heat by radiational cooling of Earths surface or by the air mass passing from a warm surface to a cold surface. The air mass may gain heat by solar heating of the ground over which the air mass moves or by the air mass passing from a cold to a warm surface.

Moisture Increase or Decrease. Moisture may be added to the air mass by evaporation. One source of evaporation may be the precipitation as it falls through the air; other sources may be a water surface, ice and snow surface, or moist ground. Moisture may be removed from the air mass by condensation and precipitation.

MECHANICAL. Mechanical influences on air masses depend upon movement. The me-chanical process of lifting an air mass over elevation of land, over colder air masses, or to compensate for horizontal convergence pro-duces a change in an air mass. Turbulent mixing and the shearing action of wind also cause air mass modifications. The sinking of air from high elevations to relatively lower lands or from above colder air masses and the descent in subsidence and lateral spreading are also important mechanical modifiers of air masses.

The thermodynamic and mechanical influ-ences on air mass stability are summarized in figure 4-1-4. The figure indicates the modifying process, what takes place, and the resultant change in stability of the air mass. These processes do not occur independently; instead, two or more processes are usually in evidence at the same time. Within any single air mass, the weather is con-trolled by the moisture content, stability, and the vertical movements of air.

Learning Objective: Describe the trajec-tories and weather associated with the air masses that influence North America and describe the air masses of Asia, Europe, and the Southern Hemisphere.

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