Determine the conditions necessary for the formation of air masses and identify air mass source regions.

Define air mass classification and describe how the classification will change when charac-teristics modify.

Describe the trajectories and weather associated with the air masses that influence North America and describe the air masses of Asia, Europe, and the Southern Hemisphere.

North American air masses, trajectories, and weather (winter)

North American air masses, trajectories, and weather (summer)

An air mass is a body of air extending over a large area (usually 1,000 miles or more across). It is generally an area of high pressure that stagnates for several days where surface terrain varies little. During this time, the air mass takes on characteristics of the underlying surface. Prop-erties of temperature, moisture (humidity), and lapse rate remain fairly homogeneous throughout the air mass. Horizontal changes of these proper-ties are usually very gradual.

Learning Objective: Determine the condi-tions necessary for the formation of air masses and identify air mass source regions.

Two primary factors are necessary to produce an air mass. First, a surface whose properties, essentially temperature and moisture, are rela-tively uniform (it may be water, land, or a snow-covered area). Second, a large divergent flow that tends to destroy temperature contrasts and pro-duces a homogeneous mass of air. The energy sup-plied to Earths surface from the Sun is distributed to the air mass by convection, radiation, and conduction.

Another condition necessary for air mass for-mation is equilibrium between ground and air.

This is established by a combination of the follow-ing processes: (1) turbulent-convective transport of heat upward into the higher levels of the air; (2) cooling of air by radiation loss of heat; and (3) transport of heat by evaporation and conden-sation processes.

By far the fastest and most effective process involved in establishing equilibrium is the turbulent-convective transport of heat upwards. The slowest and least effective process is radiation.

During radiation and turbulent-convective processes, evaporation and condensation con-tribute in conserving the heat of the overlying air. This occurs because the water vapor in the air allows radiation only through transparent bands during radiational cooling and allows for the release of the latent heat of condensation during the turbulent-convective processes. Therefore, the tropical latitudes, because of a higher moisture content in the air, rapidly form air masses pri-marily through the upward transport of heat by the turbulent-convective process. The dryer polar regions slowly form air masses primarily because of the loss of heat through radiation.

Since underlying surfaces are not uniform in thermal properties during the year and the distribution of land and water is unequal, specific or special summer and/or winter air masses may be formed. The rate of air mass formation varies more with the intensity of insolation.

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