Changes coming to hurricane ratings scale

Saffir-Simpson Hurricane wind scale updated for 2012

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MIAMI - On May 1, the new and improved Saffir-Simpson Hurricane Wind Scale will debut, just in time for the start of the 2012 hurricane season which begins on June 1.

The change to the scale is very minor but important in order to resolve awkwardness associated with conversions among the various units used for wind speed in advisory products.

The new scale will be as follows:

  • Category 1: 74-95 mph

  • Category 2: 96-110 mph

  • Category 3: 111-129 mph

  • Category 4: 130-156 mph

  • Category 5: >156 mph

According to the National Oceanic and Atmospheric Administration, 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 increments (e.g., 100, 105, 100, 115 kt, etc.).

Some advisory products, however, require intensity to be given in units of miles per hour 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 (130mph), 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.

This is not the first time the Saffir-Simpson scale has undergone change. As recently as 2009, the surge portion of the scale was dismissed because of the devastating impacts of Hurricane Ike which was ''only'' a category 2 hurricane.

For example, Hurricane Ike in 2008 was a very large storm that made landfall as a Category 2 hurricane with a peak storm surge of 15 to 20 feet. In contrast, Hurricane Charley struck southwest Florida in 2004 as a Category 4 hurricane, but produced a peak storm surge of just 6 to 7 feet.

Herbert Saffir, a consulting engineer, and Robert Simpson, who was director of the National Hurricane Center from 1967 through 1973, developed the original scale which was a useful tool to convey the threats of tropical cyclones. Changes were made to the Saffir-Simpson Scale because storm surge values and associated flooding are dependent on a combination of the storm's intensity, size, motion and barometric pressure, as well as the depth of the near-shore waters and local topographical features. As a result, storm surge values can be significantly outside the ranges suggested in the original scale.

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