Hurricane categories tell only part of the story — here's the damage storms like Irma and Harvey can do

Rebecca Harrington

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Hurricane Irma barreling toward the eastern Caribbean, captured by satellite on Tuesday.NOAA

The hurricanes Irma and Harvey are two very different storms.

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.

Hal Needham, a hurricane scientist at Louisiana State University, argued on the weather site WXshift that a storm's category doesn't fully convey how much damage it could cause.

"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:

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The Saffir-Simpson hurricane scale, which does not include lower-level tropical storms or tropical depressions, is based solely on maximum sustained wind.

Ana Pelisson/Business Insider

Once a tropical storm crosses the wind threshold above 39 mph, it gets a name. Most storms that make landfall in the US are tropical storms, not "major" hurricanes of Category 3 and above.

Hurricanes that have hit the continental US from 1950-2011, color-coded by category.NOAA

But "storms are too complex to define by one number," Needham explained. While Harvey's strong winds on the Texas Gulf Coast caused widespread destruction, most of the devastation came when it turned into a tropical storm dumping feet of water on Texas and Louisiana.

Interstate highway 45 is submerged from the effects of Hurricane Harvey seen during widespread flooding in Houston, Texas on August 27, 2017.REUTERS/Richard Carson

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.

Homes are surrounded by floodwaters from Tropical Storm Harvey on Tuesday, Aug. 29, 2017, in Houston.AP/David J. Phillip

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 74-95 mph. They can damage the outside of homes, break large tree branches, and possibly knock down power lines, causing outages for several days. Hurricane Dolly was this rating when it hit Texas in 2008.

Hurricane Dolly caused flooding of 3 feet or more in many areas of Southern Texas.Joe Mitchell/Reuters

Category 2 hurricanes have wind speeds of 96-110 mph. Storms of this intensity can cause major damage to homes and uproot large trees. They also generate power outages that last up to weeks. Hurricane Ike was a Category 2 when it hit Texas in 2008.

Hurricane Ike's winds caused severe damage when the storm hit Texas in 2008.David J. Phillip/REUTERS

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.

Hurricane Sandy caused the deaths of hundreds of people, and cost $68 billion in damage.AP

Category 3 storms have wind speeds of 111-130 mph. But with Sandy, the storm surge did some of the worst damage. It reached nearly 8 feet in parts of the Jersey Shore and 6.5 feet around New York City. Its "superstorm" status was because it was so wide — up to 1,000 miles across.

Burned houses are seen next to those which survived fires during Hurricane Sandy in Breezy Point, Queens.Adrees Latif/Reuters

Hurricane Katrina in 2005 was the most devastating storm ever to hit the United States. 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.

Flooded neighborhoods can be seen as the Coast Guard conducts initial Hurricane Katrina damage assessment overflights August 29, 2005 in New Orleans, Louisiana. Getty Images

Category 4 hurricanes have wind speeds of 131-155 mph, uprooting most trees, and creating power outages that can last weeks or even months. Hurricane Charley was a Category 4 when it made landfall in Florida in 2004.

Mobile homes are torn apart north of Port Charlotte, Florida on Aug. 14, 2004 after Hurricane Charley moved through the area.J.Pat Carter/AP

Hurricane Andrew was one of the strongest storms ever to make landfall in the US. It was a Category 5 hurricane when it hit Dade County, Florida in August 1992. Category 5 storms have wind speeds greater than 156 mph, which can completely destroy most framed homes.

The Florida City water tower still stands over the ruins of the coastal community that was hit by Hurricane Andrew in 1992.AP

If Hurricane Irma remains a Category 5 storm when it makes landfall, it could cause power outages and leave most of the areas where it hits uninhabitable for weeks to months. Theoretically, if we extended the Saffir-Simpson Scale, Irma would be a Category 6, with wind speeds between 175-195.

Hurricane Irma barreling toward the Eastern Caribbean as a Category 5 storm, captured by satellite on Tuesday, Sept. 5, 2017.NOAA

The problem with extending the Saffir-Simpson Scale is that it's also a measurement of destruction, and Category 5 storms typically cause total destruction of buildings and utilities. Technically, categories above 5 wouldn't cause more damage because there's no more damage to be done.

DON'T MISS: Here are all the areas in Hurricane Irma's path and when the storm could arrive

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