Until this year, the idea of a storm that topped the Cat5 scale seemed to be impossible. As we enter another active phase for hurricanes, do you think NOAA might have to consider adding another category? After all, if we keep getting Cat5 level storms (even if they don't sustain at that strength), doesn't it seem logical that we might, due to changes in our environment, weather patterns, whatever, need to consider a new category?
I think this is the subject of a lively debate, but nothing has been determined, one way or the other.
Here are some thoughts extracted from an article on the topic:
The history of the scale
In 1967, the United Nations commissioned Saffir, a Florida consultant engineer, to study low-cost housing in regions of the world that were prone to tropical cyclones and hurricanes.
Saffir realized there was no way to describe the effects of a hurricane, so he developed his own five-category scale. Later, Robert Simpson, then director of the National Hurricane Center, modified Saffir's work, adding measurements for flooding and storm surge.
The result was the Saffir-Simpson Hurricane Rating scale.
A Category 1 storm begins at 74 mph and a Category 5 at 156 mph. On average, there is about a 20 mph increment in wind speed between the categories.
An extrapolation suggests that if a Category 6 were there, it would be in the range of 176-196 mph. Hurricane Wilma, which had maximum recorded wind speeds of 175 mph, would have been on the verge of breaking into this hypothetical new category.
The scale didn't include a Category 6 for two reasons.
First, it was designed to measure the amount of damage inflicted by a hurricane's winds, and beyond 156 mph, the damage begins to look about the same, according to Simpson.
"When you get up into winds in excess of 155 mph you have enough damage," Simpson said in a 1999 interview with the National Weather Log, a publication of the National Oceanic and Atmospheric Administration.
"If that extreme wind sustains itself for as much as six seconds on a building it's going to cause rupturing damages that are serious no matter how well it's engineered. So I think that it's immaterial what will happen with winds stronger than 156 miles per hour. That's the reason why we didn't try to go any higher than that," Simpson said.
Another reason is that Category 5 hurricanes are relatively rare, or at least they used to be.
"In general, I didn't expect that there would be too many hurricanes that went [above] 155 miles per hour for sustained winds," Saffir said. "The limit seems to be about 175 miles per hour and I don't know of anything that goes much over that."
Some scientists predict, however, that the intensity of hurricanes and their maximum wind speeds may be increasing and that Category 4 and 5 storms will become more common in the years to come.
Behind the beast
Ocean and atmospheric temperatures work together to determine the maximum wind speed attainable. This value is known as the "maximum potential hurricane intensity" and is calculated using a formula developed in 1998 by Emanuel, the MIT climatologist.
Based on ocean and atmospheric conditions on Earth nowadays, the estimated maximum potential for hurricanes is about 190 mph.
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.
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.
Landsea believes 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.
The fastest "regular" wind that's widely agreed upon was 231 mph, recorded at Mount Washington, New Hampshire, on April 12, 1934.