katabatic wind

Cool wind that blows down a valley on calm clear nights. (By contrast, an anabatic wind is warm and moves up a valley in the early morning.) When the sky is clear, heat escapes rapidly from ground surfaces, and the air above the ground becomes chilled. The cold dense air moves downhill, forming a wind that tends to blow most strongly just before dawn.

Cold air blown by a katabatic wind may collect in a depression or valley bottom to create a frost hollow. Katabatic winds are most likely to occur in the late spring and autumn because of the greater daily temperature differences.