The thermodynamical classification applies to the relative warmth or coldness of the air mass. A warm air mass (w) is warmer than the underly-ing surface; a cold air mass (k) is colder than the underlying surface. For example, a continental polar cold air mass over a warmer surface is classified as cPk. An mTw classification indicates that the air mass is a maritime tropical warm air mass and overlays a cooler surface.

Air masses can usually be identified by the type of clouds within them. Cold air masses usually show cumuliform clouds, whereas warm air masses contain stratiform clouds. Sometimes, and with some air masses, the thermodynamic classification may change from night to day. A particular air mass may show k characteristics during the day and w characteristics at night and vice versa.

The designators and descriptions for the classifications of air masses are listed in table 4-1-1.

When an air mass moves out of its source region, a number of factors act upon the air mass to change its properties. These modifying influences do not occur separately. For instance, in the passage of cold air over warmer water surfaces, there is not only a release of heat to the air, but also a release of some moisture. As an air mass expands and slowly moves out of its source region, it travels along a certain path. As an air mass leaves its source region, the first modifying factor is the type and condition of the surface over which the air travels. Here, the factors of surface temperature, moisture, and topography must be considered. The type of tra-jectory, whether cyclonic or anticyclonic, also has a bearing on its modification. The time interval since the air mass has been out of its source region determines to a great extent the characteristics of the air mass.

You must be aware of the five modifying fac-tors and the changes that take place once an air mass leaves its source region in order to integrate these changes into your analyses and briefings.

The difference in temperature between the sur-face and the air mass modifies not only the air temperature, but also the stability of the air mass. For example, if the air mass is warm and moves over a colder surface (such as tropical air mov-ing over colder water), the cold surface cools the lower layers of the air mass and the stability of the air mass increases. This stability extends to the upper layers in time, and condensation in the form of fog or low stratus normally occurs. (See fig. 4-1-2.)

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 sta-bility), and consequently spreading to higher layers. Figure 4-1-3 shows the movement of cP air over a warmer water surface in winter. The changes in stability of the air mass give valuable indications of the cloud types that will form, as well as the type of precipitation to be expected. Also, the increase or decrease in stability gives further indication of the lower layer tur-bulence and visibility.

An air mass may be modified in its moisture content by the addition of moisture as a result of evaporation or by the removal of moisture as a result of condensation and precipitation. If the air mass is moving over continental regions, the existence of unfrozen bodies of water can greatly modify the air mass; in the case of an air mass moving from a continent to an ocean, the modification can be considerable. In general (dependent upon the temperature of the two sur-faces), the movement over a water surface in-creases both the moisture content of the lower layers and the relative temperature near the surface.

For example, the passage of cold air over a warm water surface decreases the stability of the air with resultant vertical currents. The passage of warm, moist air over a cold surface increases the stability and could result in fog as the air is cooled and moisture is added by evaporation.

Figure 4-1-2.Passage of warm air over colder surfaces.




Figure 4-1-3.Continental polar air moving from cool continent to warm ocean (winter).

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