ROBERT SIEGEL, HOST:
Hurricane Irma is massive, powerful and on track to hit South Florida after devastating parts of the Caribbean. It could go on to do damage to Georgia and the Carolinas. We have reports on hurricane preparations elsewhere in the program. But right now we have some questions about the science of these storms, and the man to answer them is Phil Klotzbach of Colorado State University. He studies Atlantic hurricanes. Welcome to the program.
PHIL KLOTZBACH: Thanks so much for having me.
SIEGEL: And how unusual is Hurricane Irma?
KLOTZBACH: Yeah, I mean, Irma's certainly one of the most powerful Atlantic hurricanes on record. It had maximum winds of 185 mph. And it maintained those winds for 36 hours, which is phenomenal. Now, its maximum winds were slightly less than Hurricane Allen back in 1980 that had maximum winds of 190 mph. But I think what sets Irma apart from some of these other very strong storms that we've had in the past is that it formed further east. Typically the really, really strong Atlantic hurricanes form in either the Gulf of Mexico, like a Hurricane Katrina, or in the western Caribbean, like a Hurricane Wilma.
SIEGEL: So talk to us briefly about the real basics here. I mean, how do hurricanes form? And how do they gather strength?
KLOTZBACH: Typically, most hurricanes form from disturbances at the Gulf over Africa and then move out into the Atlantic Ocean. And when the conditions are just right, they can intensify into tropical storms and hurricanes. But to get a storm to develop, you need to have warm ocean waters 'cause that's fuel for the storms, typically about 80 degrees Fahrenheit or warmer. You need to have low levels of vertical wind shear. So the change in wind direction with height in the atmosphere needs to be fairly limited. And it also needs a lot of moisture in the atmosphere. Basically it helps fuel the thunderstorms that are the building blocks for the hurricane.
SIEGEL: Irma is a Category 5 right now. That's the top rung of the hurricane scale. What does that mean? Well, we say it. It's the highest level for a hurricane. But what qualifies as a hurricane 5 storm?
KLOTZBACH: Yeah, I mean, the Category 5 hurricanes are absolutely devastating. You've seen the footage coming out of some of the Leeward Islands. I mean, the damage there is catastrophic. And while the storm has weakened slightly - so it was 185 miles an hour. It's still 175 mph, which still puts it well into the Category 5 scale. And the storm is not forecast to weaken very much, so certainly something that people in Florida, in the southeast U.S. are taking very seriously.
SIEGEL: So far this season we've had Harvey, which made landfall in Texas as a Category 4, and now Irma. Two massive hurricanes plus two storms that are forming - how unusual is it to have this many storms back to back?
KLOTZBACH: It is fairly unusual to get these storms back to back. But you can have significant landfalling hurricanes within several weeks of each other. Most notably, we could even go back to 2005, where we had Hurricane Dennis in July, then Katrina in August, Rita in September and then Wilma in October.
SIEGEL: Does climate change have anything to do either with the intensity of the storms or the frequency of the storms as we're experiencing them this season?
KLOTZBACH: You know, I mean, the Atlantic, actually we've had - the last two years have generally been below normal for hurricane activity. And actually, Septembers of 2013, '14, '15 and '16 were all very, very quiet. Obviously, this is a far cry from that. It's been an incredibly active last few weeks. Historically, September is the most active month of the season. When it comes to climate change's impacts on the storm - so most the theoretical models really don't see any change in the frequency, perhaps even going down a little bit. They say maybe the storms will get slightly more intense. But I typically look at the observations. I don't do much theoretical modeling. And in the observations, it's just really too hard to say.
SIEGEL: Phil Klotzbach, thanks for talking with us today.
KLOTZBACH: Thanks so much for having me.
SIEGEL: Phil Klotzbach is a research scientist with Colorado State University's Department of Atmospheric Science. He joined us via Skype. Transcript provided by NPR, Copyright NPR.