A wind gust is a sudden, brief increase in the speed of the wind followed by a lull. According to National Weather Service observing practice, gusts are reported when the peak wind speed reaches at least 18 mph and the variation in wind speed between the peaks and lulls is at least about 10 mph. The strongest wind gust recorded occurred at Australia’s Barrow Island, with a gust of 253 mph that occurred during tropical cyclone Olivia on April 10, 1996.
Gusts at the ground are caused by either turbulence due to friction, wind shear or by solar heating of the ground. These three mechanisms can force the wind to quickly change speed as well as direction.
In the case of friction, gusts are generated when wind blows around buildings, trees or other obstacles. This type of gustiness is generally largest near tall buildings and alley ways and least over large water bodies.
But air over water can still be gusty. This can be caused by wind shear. A wind shear is a change in the wind over a distance. This can be a change in wind direction, wind speed or both.
On sunny days wind gusts can be generated by rising air currents when the ground is heated. This can generate a thermal of warm air that rises, with air from above sinking to replace the rising thermal. This descending air can cause wind gusts.
The duration of a gust is usually less than 20 seconds. Whatever the mechanism, wind gusts often do things that you’d rather didn’t happen — like destroy an umbrella or blow your new hat blown away.