According to Byers, an air mass may undergo changes because of any one or more than one of the following mechanisms:-
(1) Turbulent mixing caused by eddies or convection.
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(2) Large-scale dynamic effects on lapse rate: divergence; convergence.
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(3) Sinking: in subsidence and lateral spreading; movement down from above colder air masses; descent from high elevations to low lands.
(4) Lifting: over colder air masses; to compensate for horizontal convergence; over elevations of the land.
(5) Advection of new properties aloft due to shearing action of the wind.
Mechanically induced turbulence resulting from frictional effects at the surface may cause thorough mixing, often to a considerable height. Heat and moisture are transferred from the surface to various layers of the atmosphere, thus modifying the air mass considerably.
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Large scale divergences or convergences near or above the surface may cause upward or downward movement of extensive portions of the atmosphere and affect atmospheric stability. Subsidence produces stable stratification in the atmosphere, whereas convergence makes an air mass more unstable.
A descending air mass, as one descending on the leeward side of a physical barrier, becomes more stable by subsidence. Contrarily, lifting of an air mass as in the case of an orographic uplift or a frontal uplift renders it more unstable.
In other words, ascending air is accompanied by a steepening of the lapse rate which makes an air mass unstable. Besides the above mentioned vertical motions, advection of warm or cold air aloft may affect the stability of an air mass.
Taking into account these upper level factors Petterssen has suggested the following classification of air masses: (1) s air masses which have upper-level stability, and (2) u air masses which have upper-level instability, s air masses are always found in those regions of the earth which are dominated by anticyclones.
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In these regions there is advection of warm air in the upper atmosphere, air masses, on the contrary, are normally found in cyclonic regions where there is advection of cold air aloft.