On Saturday, the United States passed the 10-year mark since its most recent major hurricane landfall — Category 3 or stronger. The last time a major hurricane made landfall in the United States was Hurricane Wilma in October 2005, which roared ashore in southwest Florida as a Category 3 packing 120-mph winds.
That’s not to say that we haven’t seen any strong or destructive storms — Hurricane Sandy obviously comes to mind. However, in terms of the most powerful hurricanes, the coastal United States has been in something of a drought.
A study published in April noted that this landfall drought is simply a matter of luck — there’s been no lack of major hurricanes out there in the Atlantic, they just haven’t hit the U.S. coast. Now a group of tropical weather researchers, led by Robert Hart at Florida State University, argues that in addition to being a matter of luck, the major hurricane landfall drought is arbitrary — that it is based entirely on the metric used to measure hurricanes, and changing that metric changes the length of the drought.
They argue two points. The first is that wind estimates are uncertain, and shifting the wind speeds of past hurricanes up or down just a little decreases the length of the current drought, and brings up a longer, 11-year major hurricane landfall drought from 1993 to 2003. The second is that very destructive hurricanes — including Sandy and Irene — both made landfall within the past 10 years, but that these storms are mainly ignored when people talk about the historical significance of the current drought.
These are fine points to make, but they don’t necessarily make this hurricane drought meaningless.
The definition of a Category 3 hurricane — and the Saffir-Simpson Hurricane Scale itself — is far from arbitrary. The Saffir-Simpson Hurricane Scale was designed to reflect the expected damage to coastal structures when specific wind speeds are inflicted. The difference between a Category 2 and a Category 3 is particularly noteworthy: A Category 3 is when we expect to see structural damage to permanent homes, while mobile and manufactured homes are usually completely destroyed.
This damage-oriented scale is why all hurricanes with winds speeds over 156 mph are grouped into the fifth category; in terms of destruction, there’s very little difference between 156 mph and 170 mph. Total destruction is just that — total. We also see this in the Fujita scale for tornadoes, which is assigned after the fact when damage can be analyzed by experts.
And, interestingly, people seem to have an evacuation instinct for a Category 3 hurricane or stronger. A 2011 survey of South Carolina residents showed that about 77 percent would likely evacuate for a major hurricane, while just 21 percent would evacuate for a non-major hurricane.
You could argue (and people have) that the scale itself needs improvement, and that the National Hurricane Center is likely making errors here and there in wind speed estimates, especially when Hurricane Hunters are not flying into the storm. But when errors do occur, they’re just as often too high as they are too low. And that error decreases as storms approach the coast and are basically monitored round-the-clock by aircraft, weather stations and satellites.
Nonetheless, the group poses that minimum pressure might be a better metric than wind speed, because it’s measured every one millibar instead of every 5 knots. The relationship between pressure and wind is not really that simple — you can’t always say that a storm with a lower pressure is going to do more damage. Wind speed depends on the pressure gradient — the difference in pressure across a distance — and is also highly dependent on latitude. But, using this oversimplified method instead of wind speed, a couple of storms since 2005 throw a wrench in the current major hurricane drought.
“One particularly poignant example of this relationship is Sandy (2012),” the authors wrote, “whose relatively modest [wind speed] yet record low [minimum pressure] translated to one of the most economically-devastating storms in U.S. history, in large part due to the storm surge.”
That is a valid point, but Sandy’s severe damage was due in no small part to the location the storm made landfall — straight into the most populous region of the United States, with the strongest winds and highest surge pushing into New Jersey and New York City during high tide. Would we even remember Hurricane Sandy if it had made landfall in Florida instead of the Northeast? Probably not — it would have been a pretty humdrum storm for the Sunshine State.
The authors also use Hurricane Irene, a Category 1 landfall in 2011, as an example of a non-major hurricane that did an extraordinary amount of damage. Again this is true — but Irene’s path also made it so. The storm passed over the populous coasts of North Carolina, Virginia and Maryland as a Category 1 hurricane and continued to track straight into Brooklyn in New York City as a tropical storm. In all, 14 states from North Carolina to Maine declared states of emergency due to Irene.
The authors are undoubtedly correct in stressing that it’s not necessarily the hurricane’s wind speed but the societal aftermath that should be emphasized in the history books. And Sandy and Irene are not taken for granted — a fact that becomes very apparent in the comments of every blog post and article written on the 10-year landfall drought.
But it’s also true that the 2005 hurricane season killed approximately 4,000 people, while the 2011 and 2012 seasons combined killed about 300 people. In terms of dollar damage, a very large but relatively weak storm can do just as much harm as a more powerful one. But the deadliest storms tend to come in the form of major hurricanes.
Furthermore, since 2005, the state of Florida has been absolutely hurricane-free. Not just major hurricanes — any hurricane. According to the Census Bureau, the population in Florida has grown by two million people in the past decade, people who may have never actually had to prepare for or respond to a hurricane landfall. “This dangerous state of affairs is compounded by the potential for complacency and lack of recent experience,” Jason Samenow wrote in a story on Florida’s foreboding hurricane drought. “When hurricanes don’t strike over such a long period of time, some people may be lulled into a false sense of security and/or forget how horrible hurricanes can be.”
Is the current major hurricane landfall drought arbitrary? Maybe. Quantifying Mother Nature does tend to be an arbitrary exercise. But it’s far from meaningless, especially for those who live along the coast.