Thermodynamic processes include such effects as heating from below which decreases the vertical stability. Other thermodynamic effects include evaporation of water into the air mass from below or into the intermediate layers by precipitation from the overlying moist air layers.
According to Trewartha the extent to which an air mass is modified depends upon (a) the initial characteristic of the air mass (its temperature and moisture content), (b) the nature of the underlying surface, (c) the path followed by the air mass, and (d) the time taken to reach the point of observation.
image source: nist.gov/mml/msed/thermodynamics_kinetics/images/Thermodynamic_2.jpg
An air mass moving over a surface that is warmer than the ground temperature is bound to be heated from below with consequent steepened lapse rate and instability. These changes create the chances of condensation and precipitation. On the other hand, an air mass moving over a colder surface is cooled from below.
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This condition favours the formation of a surface inversion which increases the stability of the air mass. Under such conditions, formation of clouds and precipitation is impossibility.
From the above discussion it is obvious that as the polar air masses move out of their source regions they tend to become more and more unstable. The tropical air masses, on the other hand, undergo the second type of modification and develop an increased stability.
Capital letters W and K standing for ‘warm’ and ‘cold’ respectively represent such modifications of the initial properties of an air mass. W is indicative of a warmer air mass being cooled from its basal layers and holding no possibility of cloud formation or precipitation.
K represents an air mass that is colder than the underlying surface and that is being heated from below. Such a change makes the air mass unstable. It should be noted that these letters simply indicate the relative temperature of an air mass with respect to the underlying surface.
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Thermodynamic changes are brought about by increased evaporation. The moisture may be supplied either from the surface over which an air mass is moving, or by precipitation from an overlying layer of the air mass itself.
On the other hand, condensation or precipitation by extracting moisture from the air mass may also cause changes. Similarly, addition or loss of latent heat accompanying the processes of condensation and precipitation may also bring about thermodynamic changes in an air mass.