As an air parcel descends through the atmosphere, it moves into a region of higher ambient pressure. Assuming the process is adiabatic (no heat exchange with surroundings) and the parcel remains in pressure equilibrium with its surroundings, it is compressed, and adiabatic compression does work on the parcel, raising its temperature while reducing its volume.
Atmospheric pressure increases toward the surface, so as an air parcel descends from higher altitude to lower altitude, the surrounding (ambient) pressure it must equilibrate...
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