New wind speed catagories proposed

Friday 29th Jul 2011 by theWeather Club

Complicated stuff wind speed. Or to be more precise, complicated stuff the way we talk about wind speed. The problem is the different scales we use to measure it, kilometres per hour (kph), miles per hour (mph) and knots (kt). It is a situation that can lead some genuine confusion in the way we define hurricanes or some ‘look the other way’ maths when conversions are being made. Something had to be done. The US National Hurricane Centre (NHC) is considering a minor change to the Saffir-Simpson Hurricane Wind Scale in order to resolve this situation, and is asking for opinions on the subject.

Currently, Category 4 is defined to be 210-249 kph which is 131-155 mph or 114-135 kt. The proposal broadens the Category 4 wind speed range by one mph at each end, giving a new range of 209-251 kph which is 130-156 mph, 113-136 kt. NHC initially assigns wind speeds in 5 knot increments which are then converted to the other two units. The resulting values are in turn rounded either up or down to the nearest 5 mph or 5 kph in the advisories that the NHC issues.

However, the Category 4 range is problematic when a hurricane's wind speed is 115 kt (132.3 mph). Although 115 kt falls within the Category 4 range, the equivalent wind speed in miles per hour rounds down to 130 mph, which falls in the Category 3 range. In order to show the hurricane as Category 4 in both sets of units, NHC must incorrectly convert 115 kt to 135 mph in its advisory products. A similar problem occurs at the current category 4/5 boundary when 135 kt is converted to kph.

Changing the Category 4 range to 130-156 mph, 113-136 kt, and 209-251 kph would allow all unit conversions from knots to be done correctly and keep storms in the correct category regardless of the units used.

The proposed changes are listed below:

Category              Existing range                   Proposed range

Cat. 1                     74-95 mph                           74-95 mph

                                64-82 kt                                64-82 kt

                                119-153 kph                        119-153 kph

Cat. 2                     96-110 mph                        96-110 mph

                                83-95 kt                                83-95 kt

                                154-177 kph                        154-177 kt

Cat. 3                     111-130 mph                      111-129 mph

                                96-113 kt                              96-112 kt

                                178-209 kph                        178-208 kph

Cat. 4                     131-155 mph                      130-156 mph

                                114-135 kt                           113-136 kt

                                210-249 kph                        209-251 kph

Cat. 5                     156 mph or higher           157 mph or higher

                                136 kt or higher             137 kt or higher

                                250 kph or higher             252 kph or higher