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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.
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.
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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