When it comes to hurricanes, the worst-case scenario just keeps getting worse.
A scientific study published this week makes a good case for revising the current scale of classification for tropical cyclones, although it stops short of actually proposing adding another category at the top.
The idea of creating a Category 6 on the Saffir-Simpson Hurricane Wind Scale has been floating around for a few years as the storms that batter coastal communities become more and more severe.
Currently, the scale recognizes five classifications:
- Category 1, sustained winds 74-95 mph
- Category 2, sustained winds 96-110 mph
- Category 3, sustained winds 111-129 mph
- Category 4, sustained winds 130-156 mph
- Category 5, sustained winds 157 mph or higher
The open-ended last category reflects the experience of forecasters at the time the scale was established in the early 1970s — that any storm with winds substantially higher than 157 miles per hour was very unlikely to materialize.
But things have changed.
In their study “The Growing Inadequacy of an Open-Ended Saffir-Simpson Hurricane Wind Scale in a Warming World,” co-authors Michael F. Wehner and James P. Kossin suggest what they call a “hypothetical” Cat 6, which would kick in when a storm reached an intensity of 192 mph sustained winds.
Not only has the number of Cat 5 storms increased in the last 40 years, they say, but in the past nine years, there already have been five storms that reached (or exceeded) the 192 mph level — all of them in the Pacific — so all would qualify for the hypothetical Cat 6.
Moreover, the authors’ mathematic modeling of storm activity found that “the chances of a PI (Potential Intensity of a given storm) exceeding the Category 6 threshold (of 192 mph) have more than doubled since 1979.”
The creation of a sixth category could promote better public awareness of the fact that tropical cyclones are becoming more intense in general, but the study authors are mindful that it could have drawbacks, too.
It could unintentionally send a message that a Cat 5 storm (or a Cat 3 or 4) isn’t all that dangerous. And it could exacerbate a weakness long criticized in the Saffir-Simpson scale, that it over-emphasizes the importance of wind-related storm consequences and downplays the effects of rain and storm surge.
The authors reference a previous study that concluded that only eight percent of tropical cyclone-related deaths are directly caused by wind, while 27 percent are because of rain flooding and a whooping 49 percent happen because of coastal storm surge.
It’s “widely believed,” they say, that there’s a need to change the messaging around hurricanes “to better inform the public about inland flooding and storm surge.”
Saffir-Simpson has already been modified several times.
A civil engineer named Herbert Saffir first developed a scale in 1971 in response to the need for insurance companies to measure risk from wind damage; meteorologist Robert Simpson adjusted the scale to account for storm surge and flooding and it was introduced to the public in 1973.
In 2009, factors related to pressure and storm surge were factored out of the scale to be delineated separately, and Saffir-Simpson became solely a wind scale.
In 2012, the Category 4 level was altered by one mph in each direction, and other categories were adjusted accordingly.
The study appeared Monday in the Proceedings of the National Academy of Sciences (PNAS), which is a peer-reviewed journal of the National Academy of Sciences.
Its co-authors are established researchers. Wehner is affiliated with the Applied Mathematics and Computational Research Division of the Lawrence Berkeley National Laboratory, Berkeley, California. Kossin is affiliated with the First Street Foundation of Brooklyn, New York, and with the Space Science and Engineering Center at the University of Wisconsin, Madison, Wisconsin.
