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Request for Comments - Proposed Modification to SSHWS

Proposed Modification to the Saffir-Simpson Hurricane Wind Scale

The National Hurricane Center is considering a minor change to the Saffir-Simpson Hurricane Wind Scale in order to resolve awkwardness associated with conversions among the various units used for wind speed. Currently, Category 4 is defined to be 131-155 mph (114-135 kt, 210-249 kph). The proposal broadens the Category 4 wind speed range by one mph at each end of the range, giving a new range of 130-156 mph (113-136 kt, 209-251 kph). Because NHC assigns intensity using 5-kt increments, neither storms in the historical record nor any future storms would have their SSHWS category changed as a result of this adjustment.

NHC's assigned intensities in 5-kt increments are converted to other units in advisory products, and the converted intensities are also rounded to the nearest 5 mph or 5 kph. However, the current Category 4 range is problematic when a hurricane's intensity is 115 kt (132.3 mph). Although 115 kt falls within the Category 4 range, the equivalent wind speed in miles per hour rounds 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.

Summary of the proposed change:

Category	Existing range		Proposed range
1		74-95 mph		74-95 mph
		64-82 kt		64-82 kt
		119-153 kph		119-153 kph

2		96-110 mph		96-110 mph
		83-95 kt		83-95 kt
		154-177 kph		154-177 kt

3		111-130 mph		111-129 mph
		96-113 kt		96-112 kt
		178-209 kph		178-208 kph

4		131-155 mph		130-156 mph
		114-135 kt		113-136 kt
		210-249 kph		209-251 kph

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

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