What wind condition defines a gust?

This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

Dear Tom,
You occasionally mention wind gusts in your weather explanations. What wind condition defines a gust?
Bill Haglund, Crete​
Dear Bill,
A wind gust is defined as a rapid fluctuation in wind speed, usually lasting less than 20 seconds. According to weather observer Frank Wachowski, there must be a variation of at least 10 knots (12 mph) between the peaks and lulls of the wind speed to report a gust. Wind gusts tend to be higher in unstable situations when cold air moves over warmer ground or over water.
Frequently, during outbreaks of cold air in the autumn or winter, wind gusts may exceed the average wind speed by 25 or 30 mph. During thunderstorms, rain-cooled air descending from a cumulonimbus cloud may generate gusts of 100 mph.
Chicago’s highest gust is 88 mph, recorded at Midway Airport on Aug. 26, 1965, at 3:45 am.