Hurricanes are categorized by several methods, the most well known being the Saffir-Simpson Hurricane Scale. The Saffir-Simpson scale, developed in 1969, is based on the wind speed of the hurricane. The various categories are listed below in their traditional form:

Category 1: 74 to 95 mph
Category 2: 96-110 mph
Category 3: 111-130 mph
Category 4: 131-155 mph
Category 5: greater than or equal to 156 mph
The amount of damage between each category does not increase linearly. Each increase in category greatly increases the damage potential. The original scale included no Category 6 hurricane. Robert Simpson has stated that there was no reason to include an additional category because the damage would be severe no matter the wind speed over 155 mph. Improvements in building engineering over the past roughly 35-40 years may have changed that dynamic and might necessitate the addition of a category 6 hurricane.

In our view, if the Saffir-Simpson scale were adjusted to include another category, it would likely be as follows:

Category 1 hurricane: 74 to 95 mph
Category 2 hurricane: 96-110 mph
Category 3 hurricane: 111-130 mph
Category 4 hurricane: 131-155 mph
Category 5 hurricane: 156-175 mph
Category 6 hurricane: greater than or equal to 176 mph
The current Saffir-Simpson scale has the following wind speed bands for its categories:
Category 1: 21 mph band
Category 2: 14 mph band
Category 3: 19 mph band
Category 4: 24 mph band
Category 5: unlimited band since there is no upper limit besides atmospheric limitations

A Category 5 band would likely be a 19 or 24 mph band and therefore a category 6 hurricane would have winds greater than or equal to 176 mph (or 181 mph). The 176 mph figure seems to make the most sense based on wind speeds and engineering.

On a related note, the question has come about, what about a storm that has winds of 95.5 mph or 110.5 mph or 131.5 mph or 155.5 mph etc? It would seem that the scale does not provide for those wind speeds. A continuous mathematical restatement of the Saffir-Simpson Scale should be made with the addition of an additional category.

The Saffir-Simpson scale should be restated as:

Category 1 Hurricane: 74 to 95 mph
Category 2 Hurricane: > 95 to 110 mph
Category 3 Hurricane: > 110 to 130 mph
Category 4 Hurricane: > 130 to 155 mph
Category 5 Hurricane: greater than 155 mph
Or, alternatively:
Category 1 Hurricane: 74 to less than 96 mph
Category 2 Hurricane: 96 mph to less than 111 mph
Category 3 Hurricane: 111 mph to less than 131 mph
Category 4 Hurricane: 131 mph to less than 156 mph
Category 5 Hurricane: 156 mph or greater
Or with an additional category:
Category 5 Hurricane: > 155 to less than 175 mph
Category 6 Hurricane: 175 mph or greater
or alternatively,
Category 5 Hurricane: 156 mph to less than 176 mph
Category 6 Hurricane: 176 mph or greater

Precision is important in the sciences and hurricane forecasting is nothing if not a science. Likewise taxonomies assist in the classification and therefore the study of any phenomenon. Our view is that the Saffir-Simpson scale has a few minor areas that need correction.

Christian H F Riley

For more information on the Saffir-Simpson Hurricane Scale, see here. And the NHC/NOAA page here.
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