frost

As the air up the mountain cools, it becomes denser, heavier, and it travels downslope due to gravity. The air continues to descend into the surrounding dense air, becoming compressed. When parcel of air loses volume, it gets warmer. Since the air is quite dry, it warms at a predictable rate of 1°C per 100m of descent (dry adiabatic lapse rate). For air that is 5°C warmer than the surrounding area may have descended from 500m upslope (depending on its original temperature).