While Harvey’s record rainfall drenched southeastern Texas and western Louisiana, flooding Houston in over 4 feet of water, Irma’s winds – if they stay as strong as they were Tuesday evening – could flatten buildings, trees, and power lines on the Caribbean islands it’s threatening to devour.
At its peak, Harvey was a Category 4 hurricane on the Saffir-Simpson scale, but its weakened winds downgraded it to a tropical storm the day after it made landfall. Irma is a Category 5 monster that’s already one of the strongest Atlantic hurricanes ever recorded – and it’s still strengthening.
“Hurricanes and tropical storms throw three hazards at us: wind, rainfall, and storm surge,” he wrote. “Think of the impacts separately. Storms with weaker winds are more likely to stall and dump heavier rainfall. This shocks people, as it would seem intuitive that a Category 5 hurricane would tend to dump more rain than a Category 1 hurricane. But the opposite is true.”
Here’s a closer look at the type of damage storms like Irma and Harvey can cause:
The Saffir-Simpson hurricane scale, which does not include lower-level tropical storms or tropical depressions, is based solely on maximum sustained wind.
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Once a tropical storm’s winds exceed 39 mph, the storm gets a name. Most storms that make landfall in the US are tropical storms, not “major” hurricanes of Category 3 and above.
But “storms are too complex to define by one number,” Needham said. While Harvey’s strong winds on the Texas Gulf Coast caused widespread destruction, most of the devastation came after it was downgraded to a tropical storm, dumping feet of water on Texas and Louisiana.
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While strong winds can rip shingles off roofs and tear down power lines, flooding often causes more widespread, costlier damage — and can be more dangerous for humans, no matter what the hurricane category is.
Harvey was particularly devastating because it stalled over the Houston area, staying in roughly the same place for five days.
Category 1 hurricanes have wind speeds of 74 to 95 mph. They can damage a home’s exterior, break large tree branches, and knock down power lines, causing multiday power failures. Hurricane Dolly was this rating when it hit Texas in 2008.
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Category 2 hurricanes have wind speeds of 96 to 110 mph. Storms of this intensity can cause major damage to homes and uproot large trees. They also generate power failures that last up to weeks. Hurricane Ike was a Category 2 when it hit Texas in 2008.
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While a hurricane’s category classifies how strong it is, this definition can’t fully predict how devastating it might be. Superstorm Sandy hit Category 3, but by the time it made landfall in New York and New Jersey in 2012 it had weakened to a post-tropical cyclone.
Category 3 storms have wind speeds of 111 to 130 mph. But with Sandy, the storm surge, or rise in sea level, did some of the worst damage. It reached nearly 8 feet in parts of the Jersey Shore and 6 1/2 feet around New York City. Its “superstorm” status was because it was so wide — up to 1,000 miles across.
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Hurricane Katrina in 2005 was the most devastating storm ever to hit the US. It killed 1,833 people and caused $108 billion in damage, though it was technically a Category 3 when it made landfall in Louisiana with sustained wind speeds of 125 mph.
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Category 4 hurricanes have wind speeds of 131 to 155 mph, uprooting most trees and creating power failures that can last weeks or even months. Hurricane Charley was a Category 4 when it made landfall in Florida in 2004.
Hurricane Andrew was one of the strongest storms ever to make landfall in the US. It was a Category 5 hurricane when it hit Florida’s Dade County in August 1992. Category 5 storms have wind speeds greater than 156 mph, which can destroy most framed homes.
If Hurricane Irma remains a Category 5 storm when it makes landfall, it could cause power failures and leave areas where it hits uninhabitable for weeks or even months. Theoretically, if we extended the Saffir-Simpson scale, Irma would be a Category 6, with wind speeds of 175 to 195 mph.
The problem with extending the Saffir-Simpson scale is that it’s also a measurement based on destruction, and Category 5 storms typically destroy buildings and utilities. Technically, categories above 5 wouldn’t cause more damage because there’s no more damage to be done.