Editor’s Note (9-5-17): As Hurricane Irma rapidly intensifies, with winds exceeding 180 mph, Scientific American reviews some of the science involved in predicting, tracking and understanding these massive storms. This article—originally published on 08/23/2011—looks at whether we will soon be facing Category 6 hurricanes.
Atmospheric researchers tend to agree that tropical cyclones of unusual ferocity are coming this century, but the strange fact is that there is no consensus to date on the five-point scale used to classify the power of these anticipated storms. In what may sound like a page from the script of the rock-band spoof Spinal Tap with its reference to a beyond-loud electric guitar amplifier volume 11, there is actually talk of adding a sixth level to the current Saffir-Simpson hurricane scale, on which category 5 intensity means sustained winds higher than 155 miles per hour (250 kilometers per hour) for at least one minute, with no speed cap.
The lack of an upper limit on the scale results in all of the most intense tropical cyclones getting lumped together, despite their wide range of power. Category 5 becomes less descriptive when it includes 2005's Emily, which reached peak wind speeds of 257.5 kph (160 mph) and six hours in category 5; the same year's Katrina which held peak wind velocity of 280 kph (175 mph) for 18 hours in the category; and 1980's Allen, churning with peak winds at 305 kph (190 mph) maintained for 72 hours in the highest category.
And now the ferocity forecast for the century adds to this classification problem. "The severe hurricanes might actually become worse. We may have to invent a category 6," says David Enfield, a senior scientist at the University of Miami and former physical oceanographer at the U.S. National Oceanic and Atmospheric Administration (NOAA). This new level wouldn't be an arbitrary relabeling. Global satellite data from the past 40 years indicate that the net destructive potential of hurricanes has increased, and the strongest hurricanes are becoming more common—especially in the Atlantic. This trend could be related to warmer seas or it could simply be history repeating itself. Data gathered earlier than the 1970s, although unreliable, show cycles of quiet decades followed by active ones. The quiet '60s, '70s and '80s ended in 1995, the year that brought Felix and Opal, among others, and resulted in $13 billion in damages and more than 100 deaths in the U.S.
The pros and cons of categories: Five or six?
The average difference between the current categories equals nearly 20 mph, so a category 6 label would likely be applied to hurricanes with sustained winds over (280 kph) 175 mph. The speed and destruction of hypothetical "category 6" storms is speculative, despite the hurricanes with winds at that level.
After all, meteorologists and climate researchers may not even choose a category 5 storm from the record books if asked to identify the most powerful tropical cyclone in history, because the Saffir–Simpson scale fixates on maximum wind speed lasting for at least one minute and disregards the many other large-scale components that factor into a storm's level of devastation. The whole index should be thrown out the hurricane-proof window, some say.
"If I could do it, I would do away with categories," says Bill Read, director of NOAA's National Hurricane Center (NHC). "The whole indexing [of hurricanes] was done back in the '60s and '70s when we had no way to convey the variables of damage that the storm did. We didn't measure it that carefully; we didn't have the tools."
Even nowadays, instruments to measure actual wind speed are often destroyed during extreme storms, so estimates have to be extrapolated from satellite images and other data. Actual observations can also be suspect. It took 14 years for the World Meteorological Organization to acknowledge that an anemometer in Australia recorded a world record wind speed of 407 kph (253 mph) during Tropical Cyclone Olivia in 1996. Wind speed science has improved over the years. Since the 1990s direct wind measurements from hurricane-hunter aircraft have replaced central pressure measurements, which were often a proxy for wind speeds.
Variables used by meteorologists and climatologists to assess damage can go beyond wind speeds to include duration over land and the extent of deadly storm surges. Read sums it up this way: "Size matters: Katrina, Rita, Ike—all of them made landfall at a 2 or 3 level, but look at the damage they caused. Obviously a category did not accurately describe the impact."
A transition to "impact forecasting" began last year when NOAA's National Hurricane Center simplified the Saffir–Simpson hurricane scale and renamed it the Saffir–Simpson hurricane wind scale. This change involved stripping away the scale's former central pressure, flooding and storm surge estimates. These factors among others are now forecast separately. In 2009 the National Weather Service began using new probability models that provide storm surge estimates ranging from 0.6 to 7.6 meters (two to 25 feet).
What the future holds
History keeps us guessing about where and when the next big tropical cyclone will hit on the U.S. Atlantic or Gulf coasts. As for the most powerful hurricane ever, experts are divided. Some say 1998's Gilbert.; an official answer from a NOAA Web site lists three: 1969's Camille, 1980's Allen and 2005's Wilma (the World Meteorological Organization agrees with the latter).
William Gray, professor emeritus of atmospheric science at Colorado State University in Fort Collins and the "grandfather" of annual hurricane season forecasting, picked the category 4 Great Miami Hurricane of 1926. NHC Director Read went with an unnamed Caribbean hurricane from 1780.
The Atlantic hurricane season, which runs from June 1 to November 30 annually, is predicted to produce more and stronger storms than average this year, although active years have been the norm since 1995. That year the Atlantic entered a period of warm sea-surface temperatures of what is called the Atlantic Multi-Decadal Oscillation, and such cycles typically last two to three decades.
"If the future is like the past, we should have another 10 to 15 years of this active period," Gray says.
This oscillation means the Atlantic is expected to cool in the future, obscuring links among hurricane activity and global warming. Perhaps counterintuitively, recent computer modeling studies predict fewer tropical cyclones if the ocean heats up further as a result of global warming. But they also predict intensification of the ones that do form, albeit with limited confidence. Frequency drops by 6 to 34 percent this century, according to 2010 review article in Nature Geoscience, whereas intensity rises 2 to 11 percent. (Scientific American is part of Nature Publishing Group.)
Today, water is a bigger concern than the wind when it comes to property destruction and loss of life. Look for more emphasis on storm surges in future forecasts, because it is the main reason why evacuations become necessary. Many planners suggest following Read's prescription: "In the U.S. 'Run from the water, hide from the wind' is pretty good, simple advice."
As for the addition of a new category 6, Read insists it is not needed. "I'd be totally opposed to that, even if they did get stronger," he says. "I'll fight 'em tooth and nail under my regime. We'll keep what we have now, but I'm going to focus more on the impacts."