Sunday, March 25, 2012

Minor Changes Coming To Saffir-Simpson Hurricane Wind Scale

The Saffir-Simpson Hurricane Wind Scale (SSHWS) is undergoing a minor modification
for 2012 in order to resolve awkwardness associated with conversions among the various
units used for wind speed in advisory products. The change broadens the Category 4
wind speed range by one mile per hour (mph) at each end of the range, yielding a new
range of 130-156 mph. This change does not alter the category assignments of any
storms in the historical record, nor will it change the category assignments for future
storms. The reasoning behind this change and a tabulation of the old and new scales is
given below.

Because of the inherent uncertainty in estimating the strength of tropical cyclones, the
National Hurricane Center and the Central Pacific Hurricane Center assign tropical
cyclone intensities in 5-knot (kt) increments (e.g., 100, 105, 100, 115 kt, etc.). Some
advisory products, however, require intensity to be given in units of mph and kilometers
per hour (km/h). For these products, the intensity in knots is converted into mph and
km/h and then rounded to 5-mph and 5-km/h increments, so as not to suggest that the
intensity of the storm can be known to unrealistic precision (e.g., 127 mph!).

Unfortunately, this conversion and rounding process doesn’t work well at the Category 4
boundaries. Category 4 has historically been defined to be 131-155 mph (with
corresponding ranges in other units given as 114-135 kt, and 210-249 km/h). A hurricane
with an assigned intensity of 115 kt, therefore, is a Category 4 hurricane. However,
when 115 kt is converted to mph (132.3 mph) and then rounded to the nearest 5 mph (130
mph), the result falls in the Category 3 mph range. In order for the hurricane to appear
as Category 4 in both kt and mph, NHC is forced to incorrectly convert 115 kt to 135
mph in its advisory products. A similar problem occurs when the Category 4 intensity of
135 kt is converted to km/h.

To solve these rounding issues, the new SSHWS broadens the Category 4 wind speed
range by one mph at each end of the range, yielding a new range of 130-156 mph (113-
136 kt, 209-251 km/h). With this change, a 115-kt Category 4 hurricane can have its
intensity properly converted to mph and rounded to the nearest 5 mph (130 mph) – and
remain within the Category 4 mph range.

It’s important to reiterate that because NHC assigns intensity using 5-kt increments (and
will be doing so for the foreseeable future), neither storms in the historical record nor any
future storms would have their SSHWS category changed as a result of this adjustment.
Changing the Category 4 range to 130-156 mph, 113-136 kt, and 209-251 km/h simply
allows all unit conversions from knots to be done correctly and keep storms in the correct
category, regardless of the units used.
Summary of the changes (highlighted in yellow):
Category >>>Previous range>>>New range
1...74-95 mph....New: 74-95 mph
2...96-110 mph....New: 96-110 mph
3...111-130 mph....New: 111-129 mph
4...131-155 mph....New: 130-156 mph
5...156 mph or higher....New: 157 mph or higher


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