Katabatic Winds

At night, particularly under calm conditions and clear skies, heat radiates from the ground, causing it to quickly cool. The adjacent lowest layers of the atmosphere, in contact with the ground, are then progressively chilled. Because it is heavier than warm air, this dense, cold air tends to flow down slopes, collecting within valleys and hollows. This flow of cold air at night is called a Katabatic Wind.

Katabatic Winds