WeatherWhys Blog

Changes come to a famous hurricane rating scale

Changes come to a famous hurricane rating scale

The first substantive change to the Saffir-Simpson scale, which rates potential hurricane damage, is coming to the world beginning May 15. This should not be an earth shattering amendment to the current category 1 through 5 assessment of hurricane strength. But it does represent the first time such a change has taken place in the 40 year history of this scale. In the early 1970s, Bob Simpson was the director of the National Hurricane Center. He and an engineer by the name of Herbert Saffir worked on the task of developing an easy to understand scale, like the Richter Scale for earthquakes, that would provide an intensity guide to potential destructive power of tropical cyclones. At the time, Saffir had been working for several years on a study commissioned by the United Nations regarding poor housing along coastlines frequented by hurricanes. By 1972 the Saffir-Simpson scale was introduced and became well known by the public at large in 1973.

Everyone today is familiar with the broad meaning of a category one hurricane versus a category five. Most Americans, if asked how strong Hurricane Katrina was that hit New Orleans in 2005, would likely connect with the fact that it was a category five storm- the worst possible. Since there is public understanding of the primary basis of this 5 part system, it seems important to mention that its values will be changing this coming hurricane season- very slightly.

The main reason owes to a common practice of rounding off wind speed values in routine hurricane bulletins. And it really only applies to one special spot where a 115-knot wind is indicated. There are a variety of users (public, marine, aviation, agriculture, etc) and a panoply of countries that use hurricane bulletins (the US, Mexico, Canada, etc), in which wind speeds are expressed as either miles per hour (mph), knots (kts) or kilometers per hour (kph). In those bulletins a wind speed is first expressed in knots and then converted to mph and kph- in each case rounded to the nearest 5 knots, mph or kph. Well, for 115 knots the actual direct conversion is 132.2 mph, but it has always be mis-converted up to 135 mph so that the storm is defined as a category 4 in both instances. Starting in May, the official category 4 values will run from 130 to 156 mph and from 113 to 136 knots. Believe it or not, that will fix a problem that you probably didn’t know existed.

The last time a long time meteorological scale changed happened in November 2001. The wind chill index, which translates an additional cooling effect of wind on exposed skin, was reevaluated. What had been a wind chill index temperature of -40 was now -15. That didn’t mean a certain temperature and wind speed combo felt any different, but the mathematical expression was calculated differently. And although a 130 mph wind speed will now be called “Category Four” hurricane, I suspect the practical difference will be hard to notice.