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Harvey approached the Texas coast as a monster Category 4 hurricane, but most of the damage came after it calmed down to a tropical storm. That damage was from the rain. NPR's Christopher Joyce reports that scientists say the traditional measure of hurricane strength doesn't tell you much about how damaging it will be, so they're proposing a new way.
CHRISTOPHER JOYCE, BYLINE: Hurricanes come in numbers - Category 3 or 4. Those numbers are set by the Saffir-Simpson scale of hurricane strength. It measures wind speed over short periods of time. The faster the wind, the higher the category. But it doesn't say much about how long strong winds will last or how they'll move. A new scale, the Cyclone Damage Potential Index, does, and it predicted that Harvey would be very bad.
JAMES DONE: I would say it's in the top 10 percent of historical events.
JOYCE: James Done is a scientist at the National Center for Atmospheric Research in Colorado. He helped develop this new index.
DONE: The index measures wind speed but also how long those strong winds blow for. So incorporates the size of the storm and how fast it's moving forward.
JOYCE: Harvey was ambling along at about walking speed for a long time, all the while dumping 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.
DONE: It has a stronger relationship to basically how long the storm sticks around for. You know, it's going to rain for a longer time.
JOYCE: Done helped to develop the new index for the insurance industry. They want better predictions about how much storms will cost them.
DONE: They can see that for storms that stall like Harvey that bring strong winds for a long period - that can actually drive up losses.
JOYCE: 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. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.