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Crane collapse

This photo taken and provided by Michael Santana shows a construction crane toppling on an apartment building Sunday as it was buffeted by high winds during a storm in Dallas, Texas. A woman was killed.

Q What forces are in play when high winds knock down objects?

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.

The wind is air in motion. The atmosphere is made up of gas molecules, mostly nitrogen and oxygen molecules.

These gas molecules are constantly in motion and exert a force when they strike an object, like our bodies or tree branches.

The force exerted by the molecules is a function of the speed, number and mass of the molecules.

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 knocking them over. Winds moving over and around objects can cause pressure changes around the object, which can also cause it to move.

The molecules that make up the atmosphere are always in motion, and some are always striking your body.

Because these molecules are moving in all directions, this force is exerted in every direction, and thus there is a balance of forces and the object doesn’t move.

"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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