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Ask the Weather Guys: How strong a wind will knock someone over?
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Ask the Weather Guys: How strong a wind will knock someone over?

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WIND AND WAVES

Ning Gao, left, and Xiaoying Li contend with blustery conditions while visiting James Madison Park in Madison, Wis., to view Lake Mendota's wind-riled waves.

How strong a wind will knock someone over?

A: Wind is air moving from areas of high atmospheric pressure to low pressure. Violent destructive winds, as well as gentle summer breezes, result from a complex interplay of different forces.

One of these forces results from a pressure gradient, or how fast pressure changes over distance. When pressure changes rapidly over a small distance, the pressure gradient force is large. Strong winds almost always result from large pressure gradients.

The greater the difference in pressure over a specific distance, the faster the air flows. Strong winds can also flow out from thunderstorms.

Since wind is air in motion, it has momentum. This momentum is transferred to the object the wind hits. Thus, the force of the wind can push objects by moving them or even knocking them over. Winds moving over and around objects can cause pressure changes around the object, which can also cause it to move.

What wind speed would knock you over?

You can derive a mathematical equation to answer that. It would depend on several factors: the velocity of wind (actually the square of the velocity); gravity; static friction (the force that keeps you anchored to the ground, along with gravity); drag of the wind pushing on you; the air density; your weight, size and center of gravity.

If you weighed 100 pounds, it would take a wind speed of about 45 mph to move you, but not knock you down, unless you lose your balance. Knocking you down would take a wind of at least 70 mph. The terminal velocity, which is the wind speed (falling speed) where the force of the wind equals the force of gravity, for a person is about 120 mph — that would likely knock you down.

"Weather Guys" Steve Ackerman and Jonathan Martin are professors in the University of Wisconsin-Madison department of atmospheric and oceanic sciences.

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