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Monday, September 1, 2008

How Strong Can a Hurricane Get?

Hurricane Gustav, churning toward the Gulf Coast now, has a small chance of becoming a Category 5 storm before it makes landfall, according to the National Hurricane Center. That would put its winds at 156 mph or stronger. Such winds would devastate most buildings and trees in the storms path. Little would be left standing.

There is no such thing as a Category 6 storm, in part because once winds reach Category 5 status, it doesn't matter what you call it, it's really, really bad.

Category 5 on the Saffir-Simpson hurricane scale has no upper bound, on paper. But in theory, winds from a powerful hurricane could blow the scale out of the water, scientists say.

Pushing the limit

The scale starts with a Category 1, which ranges from 74 to 95 mph. A Category 5 storm has winds of 156 mph or stronger. An extrapolation of the scale suggests that if a Category 6 were created, it would be in the range of 176-196 mph.

Hurricane Wilma, in 2005, had top winds of 175 mph.

How much higher could hurricane winds blow? A hurricane gains strength by using warm water as fuel. With Earth's climate warming, oceans may grow warmer, too. And so, some scientists predict, hurricanes might become stronger.

But physics dictates there must be a limit. Based on ocean and atmospheric conditions on Earth nowadays, the estimated maximum potential for hurricanes is about 190 mph, according to a 1998 calculation by Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology.

This upper limit is not absolute, however. It can change as a result of changes in climate. Scientists predict that as global warming continues, the maximum potential hurricane intensity will go up. They disagree, however, on what the increase will be.

200 mph or more

Emanuel and other scientists have predicted that wind speeds — including maximum wind speeds — should increase about 5 percent for every 1 degree Celsius increase in tropical ocean temperatures.

Chris Landsea, a meteorologist at the National Hurricane Center, disagrees.

After Wilma, Landsea said that even in the worst-case global warming scenarios, where global temperatures ratchet up by an additional 1-6 degrees Celsius, there would be about a 5 percent change, total, by the end of the 21st century. That means that hurricane-force winds are unlikely to exceed 200 mph, Landsea said.

However, Typhoon Nancy in 1961, in the Northwest Pacific Ocean, was said to have maximum sustained winds of 215 mph, according to the World Meteorological Organization's Commission on Climatology, a new clearinghouse for climate records set up at Arizona State University to settle the many disputes on weather and climate extremes. (A typhoon is the same thing as a hurricane, just in a different part of the world.)

There are known records for wind speeds that outstrip anything ever measured in a hurricane. The fastest "regular" wind that's widely agreed upon was 231 mph, recorded at Mount Washington, New Hampshire, on April 12, 1934. During a May 1999 tornado in Oklahoma, researchers clocked the wind at 318 mph.

Fix the scale?

Shortly after Wilma topped out in 2005, Emanuel called the Saffir-Simpson scale irrational, in part because it deals only with wind, ignoring factors such as a storm's size, rainfall potential and forward speed. "I think the whole category system needs serious rethinking," Emanuel told LiveScience then.

But Herbert Saffir, co-creator of the scale, countered that his scale was useful because it was simple. "As simple as it is, I like the scale," Saffir said in a post-Wilma telephone interview. "I don't like to see it too complex."

Here's why no Category 6 was included: The scale was designed to measure the amount of damage inflicted by winds, and beyond 156 mph, the damage begins to look about the same, according to Simpson.

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