Is the Saffir-Simpson Hurricane Scale Outdated (When are Statistics too Simple)?

The Saffir-Simpson scale has been used to classify  the intensity of hurricanes since the 1960s. It is based purely on the highest sustained wind speeds measured in the hurricane. So a category 1 hurricane has sustained winds between 74 mph and 95 mph; a category 2 has wind speeds between 96 mph and 110 mph, etc. It is elegantly simple, and has worked well for 40+ years.

But is it enough?

As of September, 2012, of the top 10 hurricanes (in terms of damages) in US history, three were category 1 storms; the costliest hurricane was Katrina, a category 3. Granted, the greatest damage and loss of life caused by Katrina was due to levee failure, but Katrina was at one point a category 5 as it hovered over the warmest water in the Gulf (the “Loop current”). Certainly it retained a certain percentage of that highly-charged moisture as it moved onshore, even if the wind speeds had slowed somewhat.

This summer, hurricane Isaac hit the United States as a category 1. Some residents of southern Louisiana were quoted as saying that Isaac did far more damage to their homes in Plaquemines parish than Katrina, seven years previously. Six days later Isaac had been downgraded to a “depression” located hundreds of miles away; a single band from the remaining storm dumped over four inches of rain over parts of Alabama. Dozens of tornado watches have been issued over the eastern United States during the last two days.

Update: In November, 2012, a “superstorm” hit the eastern United States: hurricane Sandy. Meteorologists were rushing to call this a unique storm, not a tropical storm but not just a nor’easter either. We had ample warning of the dangerous nature of this storm, and the alarm was justified. Forecasters avoided calling it a “category 1 hurricane” in order to emphasize the unusual potential of Sandy, thus giving support to the inadequacy of our current scale.

With all the new tools developed, and our increased ability to predict, we are still using a classification scale that is based solely on one dimension of a hurricane—wind speed. We now have the ability to gather data on numerous parameters of a storm that affect its potential impact. We have pinpoint measures of the tides (and weather conditions) at scores of offshore buoys placed all around the United States, particularly in the Gulf of Mexico. (A live map of the stations can be found here.) Doppler radar and other improved imaging techniques developed since the 1960s allow us a more complete picture of the energy contained by a hurricane, energy that is normally “stored” in a hurricane in the form of moisture. We have fairly precise modeling tools to determine the temperature gradients (energy potential) of the bodies of water over which hurricanes build power and force. Our dropsondes (sensors dropped from the hurricane-hunter planes) measure barometric pressure, humidity, and temperature more rapidly and accurately. For hurricane Sandy, the depression combined with a huge cold front moving across the country, the size of the wind field was incredibly large, and the timing of landfall coincided with the magnified high tides of full moon.

My point is simple: with all the data available today, perhaps it is time to have a discussion about changing our hurricane classification scale. For starters, we have a much more complete picture of the total moisture-energy (dewpoint, barometric pressure, over how much area) contained within a hurricane, and it might be helpful for us to have an easy reference rating that moves beyond merely sustained wind speed. In my opinion, such a discussion is needed as we continue to experience “climate change.”

I propose that we have a scale that assesses the Impact Factor for a projected landfall of such storms, whether tropical or not. We have such ability to forecast the potential of such killer storms that we can provide a much more accurate picture, using more than wind speed.


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