Q: What are the leading threats from hurricanes?

A: With all the news about hurricanes over the past couple of weeks we’ve been asked a lot of questions about the various threats posed by these storms.

Hurricane Harvey is a clear example of the damage that long-duration heavy rains can inflict. Hurricane Irma provides an example of the destructive power of the winds associated with these storms.

Objects in the path of strong winds bear what is known as a wind load. This wind load is the product of the area of the object times the wind pressure (equal to 0.00256 x windspeed2) times a laboratory-determined drag coefficient. For a wall the drag has a value of 2 while for a telephone pole it is only 0.8. A 10-foot by 20-foot wall subjected to 150 mph winds like those delivered by Irma in some locations would feel a wind load of 8.7 tons.

The most effective protection against structural damage to a house in the face of such winds is to shutter the windows. The primary reason to do so is to keep the strong winds outside of the dwelling. If a window breaks and the wind can come rushing in a lot more damage can be done to the structure.

The storm surge associated with hurricanes results from the piling up of water ahead of the storm by its strong winds. It turns out that the depth of the storm surge is best predicted by the strength of the storm about 18 hours before it makes landfall. This is because any changes in the intensity of the winds in those last 18 hours have too little time to impact the mountain of water that has already been produced.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

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