# 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.[1] 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.

## History

Development of the Hurricane Severity Index began in 2005 after the 2005 Atlantic hurricane season by Chris Hebert and Bob Weinzapfel, two ImpactWeather (acquired by StormGeo) meteorologists and hurricane experts. ImpactWeather (StormGeo) 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.[1]

## Components of the index

Visual comparison of Hurricane Floyd with Hurricane Andrew while at similar positions and nearly identical intensities. Floyd was, however, 3–4 times larger and posed a much greater threat.

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.

• Size (1–25 points)
• Examines the total coverage of the 39+, 58+, 74+ and 100+ mph wind fields.
• Intensity (1–25 points)
• Points assigned using the relationship between wind speed and the force exerted on an object.

### Determining size points

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.

HSI Size Points[2]
A total of 25 size points is possible.
35 kts 1–3
50 kts 1–4
65 kts 1–8
87 kts 1–10

### Determining intensity points

Wind force on an object is a squared function (twice the wind speed equals four times the wind force), thus, the intensity scale is also a square 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).