williwau

outflow wind

squamish wind

piteraq

Description

A williwaw is a sudden blast of wind descending from a mountainous coast to the sea. The williwaw results from the descent of cold, dense air from coastal mountains in high latitudes (Straight of Magellan, Aleutian Islands, Alaskan Panhandle, Greenland). 

Williwaw is considered a type of katabatic wind.

Squamish wind

A particular example of the williwaw is the squamish, a strong and often violent wind occurring in many of the fjords, inlets and valleys of British Columbia. Squamishes occur in those fjords oriented in a northeast-southwest direction where cold polar air can be funnelled westward, the opposite of how the wind generally flows on the coast. Within these fjords winds can create high windchills of up to -30C.  Squamishes lose their strength when free of the confining fjords and are not noticeable more than 15 nm offshore.

Flight safety considerations

Williwaws can produce hurricane force gusts. The strongest recorded in Alaska reached over 120 kts. 

Indicators and warnings: Pilots flying at low level in the region of fjords that funnel the air down from the high mountains to the sea should be mindful of the potential for williwaws and look for signs such as particularly choppy sea surfaces near the fjord entrance. Another possible indicator is a sudden rise in outside temperature as the air descending from a high mountain may have low relative humidity (similar to a Fohn wind).

Local knowledge and weather forecasts should alert pilots to the likelihood and location of these strong winds.