In the U.S. we rely on the Saffir-Simpson Scale to measure hurricanes; that scale gives a 5 rating to the strongest storms. Now there is consideration of a possible addition to the scale. See: Stronger storms mean new ‘category six’ scale may be needed. Traditional scale used goes only to five but strength and intensity of storms is increasing, says scientists
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How would that be calculated?
The vulnerability of the communities and environment hit by the storm should also figure in the scale.