Given that a moving parcels of air (and non-moving parcels too) must have the interior pressure the same as the environmental pressure, there will be interesting effects on the parcel when it moves from one place to another where the pressure is different. In particular, we will look at rising parcels, where the pressure is known to decrease during the ascent.

If we constrain the parcel to the same size as it was when it started out near the ground, the parcel pressure would remain constant, but would then become greater than the environmental pressure at any altitude above its original position. This imbalance causes a net outward pressure force on the boundary of the parcel.

So, the parcel expands under this net pressure force. As the parcel expands, the interior pressure decreases, and the expansion stops when the interior pressure equals the environmental pressure. Essentially, the parcel expands as it rises in order to keep the pressures equal across the parcel boundary. A sinking parcel will similarly compress to a smaller size.

This QuickTime movie shows how the temperature changes when air undergoes adiabatic expansion and compression. This is what happens inside an air parcel; thus, a rising air parcel loses temperature as long as we constrain its motion to be adiabatic.