When Hurricane Isaac swept ashore as a Category 1 storm, its destructive power seemingly took some by surprise. As images of submerged houses and news of dramatic rooftop rescues made the rounds, the Web was abuzz with expressions of disbelief that a mere "Cat 1" storm could be the culprit for the catastrophe unfolding before people's eyes.

"Man that's just too much water to be a Category 1 or tropical storm," wrote @KeepNUpWithMike, reflecting the sentiments of many Twitter users.

Yet these labels — Category 1 hurricane, tropical storm— are based only on top sustained wind speeds. And although that's important, it's by no means the only factor that determines a storm's appetite for destruction, explained Tim Schott, a meteorologist at National Weather Service headquarters in Silver Spring, Md. "There are other factors that can be critical as well," he told OurAmazingPlanet.

However, none of these other factors are used to rank hurricanes on the Saffir-Simpson Scale. And some scientists argue that since the labels reflect only one facet of a storm's destructive power, it's time to change the way we categorize tropical storms and hurricanes.

Yet all forecasters emphasized that storms that are comparatively low on the hurricane scale can still cause horrific damage.

"A lot of the news media and people in general have noted this is only a Category 1 storm, but [Isaac's effects] show that Category 1 is just the description of the winds," Schott said. [Images: Hurricane Isaac Packs a Punch]

Causes of destruction

Along with wind speed, the sheer size of a storm, how quickly it is moving and the angle at which it's approaching land also influence its potential to wreak havoc. Isaac possessed some of the most menacing of these qualities.

First, Isaac is simply a big storm, said Chris Landsea, the science and operations officer at the National Hurricane Center in Miami. "Ever since it formed, it's been a large tropical cyclone," he told OurAmazingPlanet.

Bigger storms have a wider reach, affecting more people, and they're also tied to more severe storm surge, or "the ocean being pushed ashore," as Landsea described it.

"Isaac's storm surge is extended all the way to the Florida panhandle," Schott said, adding that parts of Louisiana have seen inundation as high as 11 feet (3 meters).

Isaac is also moving slowly. The storm crept along at about 6 mph (9 kph) for most of Tuesday and Wednesday, and is moving at 8 mph (13 kph) today (Aug. 30) — a snail's pace for a storm. That means it's hammering the same areas with heavy downpours for hours, even days, at a time.

Even relatively wimpy storms can cause terrible damage if they stay parked in place, Landsea and Schott said, since the unending rains can spark inland flooding. Isaac has reportedly dropped some 23 inches (58 centimeters) of rain near Gretna, La., according to unofficial numbers from the National Hurricane Center.

The direction of a storm's approach also plays a role in its destructive power — certain angles promote greater storm surge, said Robert Henson, a meteorologist who works as a science writer for the University Corporation for Atmospheric Research, in Boulder, Colo.

3 Comments

1. 1. way2ec 08:57 PM 8/30/12

   Another factor should be in the mix, a vulnerability index. Thousands of people living below sea level and "protected" by levees, thousands of people with oceanfront properties, and as noted in this article, people who don't even bother to put up storm shutters thinking it is "just a Category 1" storm. In the future maybe we could get better warnings, as in if this huge slow moving storm bringing incredible amounts of rain makes landfall near the following areas, we expect more destruction "than normal". We are part way there now, as we do know that New Orleans is very vulnerable, thus so much more attention given to that city. If storms have "appetites" for destruction then we should label where we are the juiciest "targets".

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2. 2. Kurt Erlenbach 10:49 PM 8/30/12

   The "vulnerability index" way2ec proposes is insufficiently particularized to be useful. I live on the east coast of Florida, but Merritt Island/Cape Canaveral is between me and the ocean. My house is much more vulnerable to wind than to rain. Down the street, things are different - homes there have roofs likely to survive a 1 or 2, but a dozen inches of rain will swamp the first floor. Homeowners must learn their own vulnerabilities. The incessant hype and "crying wolf" by the local media, rather than reasoned analysis, make evaluating vulnerabilities difficult.

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3. 3. Organic1 10:32 AM 9/2/12

   Simple metrics to classify storms in general, will always lack true descriptions of characteristics for the storms. Until those that do these classifications determine better factors to place markers on them, we will always have to rely on reactive measures. When it is based on wind speed and monetary damages only, that is all we can expect to value the forecasts and/or actual results.

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