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Posted at 12:08 PM ET, 09/07/2012

Why were hurricane categories established? Behind the Saffir-Simpson scale

Most of us are familiar with hurricane categories which rate storms on a 1-5 scale, known as the Saffir-Simpson Hurricane Wind Scale. What was the motivation for the scale’s development?


Bob Simpson (NOAA)
The terrible toll of hurricane Camille in 1969 prompted Bob Simpson, who was then National Hurricane Center (NHC) director, to start thinking of better ways to communicate hurricane dangers to emergency officials.

When he looked back on how the NHC had handled Camille, Simpson said he felt he had failed to “communicate with people such as the federal Office of Emergency Planning (It became the Federal Emergency Management Agency in 1979.), the Red Cross, the Salvation Army, and state agencies that were preparing to aid victims.

“I couldn’t get a handle on the storm to answer their questions,” he said. “I didn’t know their job well and I couldn’t equate it to the central pressure of the storm, which I could tell them about. I needed something to give them a handle on it, so they would know what they would need to deal with a storm.” [Note, these two paragraphs are from p. 153, “Hurricane Watch” by Dr. Bob Sheets and Jack Williams, New York, 2001.]


Saffir-Simpson Hurricane Wind Scale (National Hurricane Center)
He began with the storm categories that Herbert S. Saffir (1917 – 2007) had devised for a United Nations report on low-cost buildings that could withstand strong winds. Saffir was an engineer who had written the Dade County (Miami) Florida building code. Simpson’s big change was adding likely storm surge heights to Saffir’s wind damage scale. But those were later removed (as discussed further down).

One reason for including possible surge in the categories was that Simpson had, for the first time, used a computer model prediction of surge height before Camille hit. This prompted emergency officials to urge last-minute evacuations that they estimated saved hundreds of lives.

After the successful use of the surge model, Simpson was confident that he and subsequent hurricane forecasters would be able to predict surge heights.

Until he retired as NHC director in 1974, Simpson used hurricane categories to help the Red Cross, local officials, and others prepare to provide sufficient aid to victims, but didn’t release them to the public.

By the time Neil Frank succeeded Simpson as NHC director in 1974, news organizations and others were clamoring for storm categories and Frank began releasing the categories to the pubic in 1975.

From the beginning Simpson, Frank, subsequent NHC directors, most broadcast meteorologists and others who inform the public about weather dangers have stressed the limitations of categories. They generally stressed that hurricane winds and storm surge are far from uniform across the storm. Probably the biggest concern has been that the categories don’t take into account of deaths and damage from “fresh-water” flooding; that is from floods caused by the heavy rain that even a tropical storm can bring.

As it turns out, even if Simpson had been using today’s Saffir-Simpson Scale for Camille it wouldn’t have helped save the 112 people killed in middle-of-the-night flash floods caused by Camille’s remnants on the east slopes of Blue Ridge Mountains in Virginia and West Virginia.

In fact, from Camille in 1969 to Katrina in 2005, freshwater flooding from inland rain was the big hurricane and tropical storm killer in the United States, accounting for more than half of the tropical cyclone-related deaths. For example, inland floods killed fifty of the fifty-six people who died in Hurricane Floyd in 1999.

Beginning in 2010, storm surge was removed from the Saffir-Simpson scale, and it was renamed the Saffir-Simpson Hurricane Wind Scale.

The motivation for deleting references to storm surge was that, while surge is strongly related to wind speed, other factors can strongly affect this sometimes deadly rise in water as hurricanes wash ashore.

As Jason Samenow described in his 2009 piece:

The surge is primarily driven by the storm’s winds, but is also impacted by the size of the storm and the geographic profile of the coast it is inundating...

For example, a hurricane striking the mid-Atlantic would have a smaller surge than the same storm along the coast of the western Gulf of Mexico which has a fairly flat coastal profile. But the Saffir-Simpson scale does not take the slope of the coast into account.

The scale also fails to take into the account the physical size of a hurricane. Generally, the larger the storm in areal extent, the bigger the surge. A very intense but compact storm may have a smaller surge than a weaker but more expansive storm. For example, the powerful, but tiny category 4 Charley (2004) -- had just a 7 foot surge when it washed ashore southwestern Florida whereas the somewhat less intense (category 3) but much more massive Katrina produced a devastating 28-foot surge along a portion of the Louisiana coast.

So for now, the Saffir-Simpson scale is simply in an indicator of a hurricane’s peak sustained winds. And as wind is just one of many hazards posed by a hurricane, many meteorologists and risk communicators worry the information it provides is incomplete at best and misleading at worst.

Our next piece will discuss possible ways to overcome the limitations of the Saffir-Simpson scale.

(Jason Samenow contributed to this piece)

By Jack Williams  |  12:08 PM ET, 09/07/2012

 
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