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 (119 to 153 km/h). A Category 5 storm has winds of 156 mph (251 km/h) 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 (280 km/h). And as of Tuesday morning (Sept. 5, 2017), Hurricane Irma's winds were raging at a whopping 175 mph (280 km/h) as well, with the potential to strengthen, according to the National Hurricane Center, part of the National Oceanic and Atmospheric Administration (NOAA). Irma is churning about 270 miles (440 kilometers) east of Antigua and 280 miles (445 km) east-southeast of Barbuda, the NHC says. [A History of Destruction: 8 Great Hurricanes]
How much faster 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. Particularly, researchers have found the strongest storms should become even more intense as the planet warms, Live Science previously reported. By the end of the 21st century, human-caused global warming will likely increase hurricane intensity, on average, by 2 to 11 percent, according to a review by NOAA's Geophysical Fluid Dynamics Laboratory, revised on Aug. 30, 2017.
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 (305 km/h), 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 due to 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 by about 5 percent for every 1 degree Celsius increase in tropical ocean temperatures. [A Guide to Hurricane Season 2017]
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 1.8 degrees to 10.8 degrees Fahrenheit (1 to 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 (322 km/h), Landsea said.
However, Typhoon Nancy in 1961, in the Northwest Pacific Ocean, was said to have maximum sustained winds of 215 mph (346 km/h), according to the World Meteorological Organization's Commission on Climatology, a 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" (or non-storm) wind that's widely agreed upon as the record-holder — 231 mph (372 km/h), recorded at Mount Washington, New Hampshire, on April 12, 1934. During a tornado in May of 1999 in Oklahoma, researchers clocked the wind at 318 mph (512 km/h).
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. For instance, Tropical Storm Harvey, which made landfall in Texas as a hurricane on Aug. 25, 2017, dumped unprecedented amounts of rain on areas of the state, causing devastation in the Houston area. One rain gauge picked up more than 51 inches of rainfall, according to NOAA.
"I think the whole category system needs serious rethinking," Emanuel told Live Science after Wilma.
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.
Editor's Note: This article was first published on Oct. 16, 2012, and then updated in 2017 with more recent hurricane information.