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Sunday, March 25, 2012

NOAA Announces Modification to Hurricane Scale

The Saffir-Simpson Hurricane Wind Scale is the standard used to measure hurricane intensity, and this year, the National Oceanic and Atmospheric Administration (NOAA) is making a modification of the scale. NASA's Hurricane Web page uses the official NOAA measurements in its tropical cyclone coverage.

In a Public Information Statement from NOAA's National Weather Service Headquarters, Washington, the minor modification of Saffir-Simpson Hurricane Wind Scale thresholds go into effect as of May 15, 2012. The scale is being adjusted slightly to resolve issues associated with the conversion of units used for wind speed.

The statement noted that "The National Hurricane Center (NHC) and Central Pacific Hurricane (CPHC) assign intensities of hurricanes in 5-knot (kt) increments. For advisory products, these intensities are converted to miles per hour (mph) and kilometers per hour (km/h), and then rounded to the nearest 5-mph or 5-km/h increments.

Challenges occur when the current Category 4 intensity is 115 kt (132.3 mph). Although 115 kt is within the Category 4 range, the equivalent rounded wind speed in mph is 130 mph, which is in the Category 3 range. To classify the hurricane as a Category 4 in both sets of units (kt and mph), NHC and CPHC must incorrectly convert 115 kt to 135 mph in the advisory products. A similar issue occurs when the current Category 4 intensity of 135 kt is converted to km/h."

As of May 15, 2012, Category 4 storms on the scale will be broadened by one mph at each end of the range, yielding a new range of 130-156 mph to resolve the issue. That change also affects the Category 3 and 5 wind speed thresholds. With this change, a 115-kt Category 4 hurricane will have its intensity properly converted to mph and rounded to the nearest 5 mph (130 mph) and remain within the Category 4 mph range.

The Saffir-Simpson Hurricane Wind Scale will change as follows:

Category 3: 111-130 mph (96-113 kt, 178-209 km/h)
Category 4: 131-155 mph (114-135 kt, 210-249 km/h)
Category 5: 156 mph or higher (136 kt or higher, 250 km/h or higher)

Category 3: 111-129 mph (96-112 kt, 178-208 km/h)
Category 4: 130-156 mph (113-136 kt, 209-251 km/h)
Category 5: 157 mph or higher (137 kt or higher, 252 km/h or higher)

There will be no change to the wind speeds currently assigned to Categories 1 and 2.

NOAA announced that none of the storms in the historical record will have their category changed as a result of this modification to the wind scale.

Additional information on this change can be found at:

Text Credit: NOAA Public Information Statement
National Hurricane Center

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