Hurricane Severity Index

The Hurricane Severity Index (or HSI) is a hurricane rating system which defines the strength and destructive capability of a storm. The HSI uses equations which incorporate the intensity of the winds and the size of the area covered by the winds. The HSI attempts to demonstrate that two hurricanes of similar intensity may have different destructive capability due to variances in size, and furthermore that a less intense, but very large hurricane, may in fact be more destructive than a smaller, more intense hurricane. HSI was developed by a private company program in competition with the National Weather Service's Accumulated cyclone energy index.

Development of the Hurricane Severity Index began in 2005 after the 2005 Atlantic hurricane season by Chris Hebert and Bob Weinzapfel, two ImpactWeather meteorologists and hurricane experts. ImpactWeather officially announced the HSI in 2006. Their goal was to create a new index that rates the severity of all types of tropical cyclones (not just hurricanes) based on both their intensity and size of wind field.

The idea behind the Hurricane Severity Index is that the size of a hurricane is as important as the strength of its winds. Thus, the index uses a 50-point scale, with half of the total based on wind intensity, and half based on the wind fields.

Wind radii data from every named storm since 1988 was studied. From these data, average wind radii ranges of four wind fields (39, 58, 74, and 100 mph) were calculated. Once the typical ranges were established, each wind field range was divided into sections. Since hurricane-force winds are much more damaging than tropical storm-force winds, the size scale is weighted more toward the 74 and 100 mph wind fields. With the HSI, a tropical storm can receive no more than 7 total points for size.

Wind force on an object is a quadratic function (twice the wind speed equals four times the wind force), thus, the intensity scale is also quadratic in point value, e.g. 1 point for a 30 kt (35 mph) tropical depression and up to 25 points for a hurricane with winds above 150 kts (175 mph).

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