Harvey approached the Texas coast as a monster Category 4 hurricane, almost as powerful as they come. But most of the damage has come after it calmed down to a mere tropical storm — from rain.
Scientists say the traditional measure of hurricane strength doesn't tell you much about how damaging it will be. Now they're proposing a new way to do that.
Hurricanes come in numbers — Categories 1 through 5 — as part of the Saffir-Simpson scale of hurricane strength. It measures wind speed over short periods of time, and the faster the wind, the higher the category.
But it doesn't say much about how long strong winds will last and how that "wind field" will move.
A new scale, the Cyclone Damage Potential index, does. And it predicted that Harvey would be very bad. "I would say it's in the top 10 percent of historical events," says James Done, an atmospheric scientist at the National Center for Atmospheric Research in Colorado, who helped develop this new index. "The index measures wind speed," he says, "but also how long those strong winds blow for. So it incorporates the size of the storm and how fast it's moving forward."
Harvey was ambling along at about walking speed for a long time, which allowed it to dump trillions of gallons of water on Houston.
By focusing more on how far and wide winds blow and for how long, the new index tells a much more complete story than the traditional scale. "It has a stronger relationship to basically how long the storm is going to stick around for," Done says, and whether it is going to rain for a longer time.
Done helped develop the new index for the insurance industry, which wants better predictions about how much storms will cost insurers. "They can see that for storms that stall like Harvey, that bring strong winds for a long period, that can actually drive up losses," Done says.
Done notes that ultimate damage losses also depend on other things, like how much property is in harm's way in a particular place. But he says the new index gives a more complete picture of what a storm can do.