Dynamic (or mechanical) changes are superficially different from thermodynamic changes because they involve mixing or pressure changes associated with the actual movement of the airmass. The physical properties of airmasses are considerably modified, for example, by a prolonged period of turbulent mixing (see Figure 5.7). This process is particularly important at low levels, where surface friction intensifies natural turbulence, providing a ready mechanism for the upward transfer of heat and moisture.
The radiative and advective exchanges discussed earlier are diabatic, but the ascent or descent of air causes adiabatic changes of temperature. Large-scale lifting may result from forced ascent by a mountain barrier or from airstream convergence. Conversely, sinking may occur when high-level convergence sets up subsidence or when stable air, that has been forced up over high ground by the pressure gradient, descends in its lee. Dynamic processes in the middle and upper troposphere are in fact a major cause of airmass modification. The decrease in stability aloft, as air moves away from areas of subsidence, is a common example of this type of mechanism.
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