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Tropical Cyclone Hazards: Wind and Squalls

Cat. Speed (mph)
1 74-95
2 96-110
3 111-130
4 131-155
5 >155
Hurricanes are known for their damaging wind. They are rated in strength by their wind also. The Saffir-Simpson Hurricane scale (right) is the rule by which their strength is rated.

However, when the NWS's National Hurricane Center issues a statement concerning the wind and catagory, that value is for SUSTAINED wind only. This hurricane scale does not include gusts or squalls. Gusts are short but rapid bursts in wind speed and are primarily caused by turbulence over land mixing faster air aloft to the surface. Squalls, on the other hand, are longer perids of increased wind speeds and are generally associated with the bands of thunderstorms which make-up the spirial bands around the hurricane.

A tropical cyclone's wind damages and destroys structures two ways. First, many homes are damaged or destroyed when the high wind simply lifts the roof off of the dwellings. The process involved is called Bernoulli's Principle which implies the faster the air moves the lower the pressure within the air becomes. The high wind moving over the top of the roof creates lower pressure on the exposed side of the roof relative to the attic side.

The higher pressure in the attic helps lift the roof. Once lifted, the roof acts as a sail and is blown clear of the dwelling. With the roof gone, the walls are much easier to be blown down by the hurricane's wind.

Plywood wedged in a palm treeThe second way the wind destroys buildings can also be a result of the roof becoming airborne. The wind picks up the debris (i.e. wood, metal siding, toys, trash cans, tree branches, etc.) and sends them hurling at high speeds into other structures. Based on observations made during damage investigations conducted by the Wind Science and Engineering Research Center at Texas Tech University, researchers realized that much of the damage in windstorms is caused by flying debris.

They found, based on damage investigations, sections of wooden planks are the most typical type of debris observed due to tornado. A 15-lb 2x4 timber plank in a 250 mph wind would travel at 100 mph. While 250 mph is considerably more than even the strongest hurricane's sustained wind, the wind in squalls and tornadoes, could easily reach that speed.

Next: Tropical Cyclone Hazards - Heavy Rain and Inland Flooding

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Page last modified: July 27, 2004
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