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In meteorology, a williwaw is a sudden blast of wind descending from a mountainous coast to the sea. The word is of unknown origin, but was earliest used by British seamen in the 19th century. The usage appears for winds found in the Strait of Magellan, the Aleutian Islands and the coastal fjords of the Alaskan Panhandle, where the terms outflow wind and squamish wind are also used for the same phenomenon. On Greenland the word piteraq is used.
The williwaw results from the descent of cold, dense air from the snow and ice fields of coastal mountains in high latitudes, accelerated by the force of gravity. Thus the williwaw is considered a type of katabatic wind.