Changes come to a famous hurricane rating scale

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.