A katabatic wind is a gravity-driven wind that flows downslope when air near a mountain surface cools (especially at night through radiative cooling), becomes denser than the surrounding atmosphere, and drains downhill under its own weight. The term comes from the Greek katabasis (descent). These winds are most common on clear nights with calm synoptic conditions, when radiative cooling is most effective.
The scale and intensity of katabatic winds varies enormously. On a local scale, gentle katabatic breezes of 5–15 km/h flow down valley sides each evening, pooling cold air in valley floors — creating frost hollows where temperatures can be 10–15 °C lower than on surrounding slopes. This phenomenon is well known to farmers and vineyard owners who site their crops to avoid cold-air pools. The opposite daytime flow — warm air rising along sun-heated slopes — is the anabatic wind.
At the extreme end, the world's most powerful katabatic winds blow off the Antarctic and Greenland ice caps, where kilometres-thick ice cools the air layer above it to extreme temperatures. Antarctic katabatic winds regularly exceed 150 km/h and have been recorded above 300 km/h at Cape Denison (Adélie Land), one of the windiest places on Earth. In Europe, the Bora of the Adriatic coast and the Mistral of the Rhône valley have strong katabatic components, plunging temperatures and battering coastal areas.