The Gathering Storm Over How Hurricanes Are Measured

(Bloomberg) -- Call anything a Category 5 storm, disaster or crisis and immediately it sounds awful. The label owes much of its weight to the Saffir-Simpson Hurricane Wind Scale, which is cited routinely (if rarely by its full name) this time of year during the Atlantic hurricane season. But destructive storms like Hurricane Florence underscore the inherent weaknesses in the scale, and efforts continue to try to replace it with something more useful.

1. What is Saffir-Simpson?

It’s a five-step scale created in the 1970s by Herb Saffir, an engineer, and Bob Simpson, a meteorologist, to categorize a hurricane’s power, from Category 1 (sustained winds of at least 74 miles per hour, or 119 kilometers per hour) to Category 5 (157 mph and greater). Anything above a Category 3, which is reached when a storm’s winds hit 111 mph, is considered a major hurricane. The scale is used primarily in the Western Hemisphere, where the U.S. National Hurricane Center has responsibility for forecasting.

2. Why does it matter?

It’s become a staple of hurricane reports by governmental agencies, emergency managers and broadcasters. Every advisory from the National Hurricane Center lists a storm’s category. If it moves one category higher, it’s often described as strengthening; if it drops, then in the public mind it has weakened. In the aftermath of Hurricane Florence, news outlets found numerous examples of people in North Carolina who thought the storm wouldn’t be so bad because it was dropping on the Saffir-Simpson scale.

3. What’s wrong with the current scale?

Critics say it focuses public attention on wind even if rain or tidal surges are a storm’s most destructive features. Florence was downgraded from Category 4 all the way to Category 1 just before it struck the U.S. East Coast but still left a trail of damage through North and South Carolina. Plus, a hurricane’s strongest sustained winds generally affect only a limited area. Hurricane Patricia, which struck Mexico in 2015, went down in history as the strongest storm recorded in the Western Hemisphere, with winds at its core reaching 200 mph. But within a space of 30 miles they dropped to 73 mph. For that reason -- and because it hit a sparsely populated region -- Patricia caused far less damage than might have been expected. Another example: Though Hurricane Harvey’s winds strengthened right before it came ashore in Texas in 2017, it was primarily rain that made it the second-most-costly storm in U.S. history. A storm’s physical size, the location it hits and the angle it approaches land also can play larger roles in how much destruction is caused.

4. Why is Saffir-Simpson limited to wind?

Each of the scale’s five categories included an expected storm surge -- 4 to 5 feet for Category 1, up to more than 18 feet for Category 5 -- until 2009, when the National Hurricane Center dropped that detail as "scientifically inaccurate." A storm surge, when a wall of water breaks on shore, causes more damage and deaths than wind.

5. Aren’t there already warnings geared to rain and tides?

Yes. The National Hurricane Center has a separate method for warning against storm surge and continuously warns the public not to fixate on wind speed alone. The National Weather Service also issues flood warnings and watches and predicts how rain and storm surges might affect rivers and streams. But none of those warnings is as pithily conveyed, or widely reported, as a storm’s movement up or down Saffir-Simpson.

6. Is there an alternative to the Saffir-Simpson scale?

There have been a number of attempts to come up with a warning system that incorporates size, storm surge, rain and wind into a single designation that can be broadcast to the public. Two meteorologists came up with the Hurricane Severity Index, a 50-point scale that aims to take into account the size of a storm and area of its strongest winds. Researchers at the National Center for Atmospheric Research in Boulder, Colorado, proposed what they call the Cyclone Damage Potential Index, a 0-10 scale that considers a storm’s intensity, size and forward speed. But no one scale can effectively communicate all the risks associated with a hurricane, said Rebecca Morss, a senior scientist at the National Center for Atmospheric Research. References to Saffir-Simpson’s well-known scale at least get people "to pay attention and get more information" about whether they need to evacuate, she said.

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