A wind gust is a sudden, seconds-long burst of high-speed wind that's followed by a lull. Whenever you see wind gusts in your forecast, it means the National Weather Service has observed or expects wind speeds to reach at least 18 mph, and the difference between the peak winds and the lulls to vary by 10 mph or more. A related phenomenon, a squall, is (according to the National Weather Service), "A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute."
Why Does the Wind Gust?
There are a number of things that disturb wind flow and make its speed vary, including friction and wind shear. Whenever wind's path is obstructed by objects such as buildings, mountains, or trees, it hugs the object, friction increases, and the wind slows. Once it passes the object and flows freely again, the speed increases rapidly (gusts).
When wind travels through mountain passes, alleys, or tunnels, the same amount of air is forced through a smaller pathway which also causes an increase in speed or gusts.
Wind shear (a change in wind speed or direction along a straight line) can also lead to gusting. Because winds travel from high (where there's more air piled up) to low pressure, you can think of there being more pressure behind the wind than in front of it. This gives the wind a net force and it accelerates in a rush of wind.
Maximum Sustained Winds
Wind gusts (which last only a few seconds) make it hard to determine the overall wind speed of storms whose winds don't always blow at constant speeds. This is especially the case for tropical cyclones and hurricanes. To estimate the overall wind speed, the wind and wind gusts are measured over some period of time (typically 1 minute) and are then averaged together. The result is the highest average wind observed within the weather event, also called the maximum sustained wind speed.
Here in the U.S., maximum sustained winds are always measured by anemometers at a standard height of 33 feet (10 m) above ground for a duration of 1 minute. The rest of the world averages their winds over a period of 10 minutes. This difference is significant because measurements averaged over just one minute are about 14% higher than those averaged over the course of ten minutes.
High winds and gusts can do more than turn your umbrella inside out, they can cause legitimate damage. Major wind gusts can knock down trees and even cause structural damage to buildings. Wind gusts s as low as 26 mph are strong enough to cause power outages.
The Highest Gusts on Record
The world record for strongest wind gust (253 mph) was observed on Australia's Barrow Island during the passage of Tropical Cyclone Olivia (1996). The second highest wind gust ever recorded (and the #1 strongest "ordinary" gust not linked to a tropical cyclone or a tornado) occurred right here in the United States atop New Hampshire's Mount Washington in 1934.