DAVID GREENE, HOST:
Hurricane Irma is now a Category 4 hurricane. That is down from a 5. Still, maximum sustained winds of 155 miles an hour. This is slightly weaker from when Irma wiped most of the structures off of islands it passed over in the Caribbean. Now, as the storm heads towards the United States mainland, Florida Governor Rick Scott warned people in evacuation zones not to ride this out.
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RICK SCOTT: Based on what we now know, Miami-Dade will have major hurricane impacts with deadly storm surge - deadly storm surge - and life-threatening winds. We can expect this all along the entire East Coast.
GREENE: Well, Nancy Masry (ph) from Key West heeded this call. She was on the Florida Turnpike when we reached her. She said she was pressured by friends and family to evacuate.
NANCY MASRY: I just got back, about a week ago, from a two-week road trip and the last thing I really wanted to do was get back in a car and live out of a suitcase.
GREENE: And here's NPR's David Schaper on how bad this storm could be.
DAVID SCHAPER, BYLINE: Imagine speeding down the highway at 70 miles an hour with your car window open.
DAVID PREVATT: And you stick your hand out and you hold it in the wind. Do you have a sense of how the wind is going to push your hand back?
SCHAPER: David Prevatt, a professor of civil engineering at the University of Florida, says, now imagine getting out of his old Mazda and getting into a Maserati going 185.
PREVATT: The force that I will feel on my hand, in comparison to that 70 mile per hour, is going to be almost seven times as strong.
SCHAPER: This is Prevatt's way of saying Hurricane Irma's top winds are exponentially stronger than anything most people have ever experienced before. Prevatt says another element to consider is the uplift, or suction force, of the winds.
PREVATT: The wind is going to try to pull the roof and roof components vertically up and away from the ground.
SCHAPER: So how well structures hold up depends, not just on the strength of building materials, but, more importantly, the strength of the connections holding roofs, walls and foundations together. What about high rises? Northwestern University civil engineering professor Joseph Schofer answers that concern.
JOSEPH SCHOFER: Tall structures are not particularly vulnerable, except for damage from flying objects. And clearly, large objects are going to boosted into the air with a wind speed like this.
SCHAPER: And Schofer says wind isn't the only concern - so, too, is water.
SCHOFER: Flooding is going to be a problem because of the rain, but the storm surge is going to create a particularly threatening problem along the coast.
SCHAPER: Schofer says a high storm surge could be stronger than wind and knock down buildings, bridges and other seaside structures. And Carol Considine, a professor of engineering technology at Old Dominion University, adds that because sea levels are higher, any storm surge that comes ashore with Irma will be that much more damaging.
CAROL CONSIDINE: We're definitely going to have roadways that may be undermined and damaged from this type of storm. You're also going to have coastal flooding which could inundate the stormwater system in communities.
SCHAPER: Dams and levees are vulnerable to a storm of Irma's strength too, as is the electric power grid. Many of these systems are more resilient now. Building codes have improved since Hurricane Andrew 25 years ago. But, Considine says, we shouldn't just improve standards based on past storms.
CONSIDINE: We need to start thinking about, what do we expect in the future and start changing the way we design to anticipate some of those future events?
SCHAPER: After all, she says, storms are getting worse, and Hurricane Irma is a prime example.
GREENE: David Schaper.
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