How to Measure a Hurricane How to Measure a Hurricane

When does a storm become a hurricane? And what do the category numbers mean as it gains in strength? Knowing how to measure a hurricane can be a key element in survival.

The force of a hurricane is measured on the Saffir/Simpson Scale, developed in the 1970s by civil engineer Herbert Saffir and US National Hurricane Center director Robert Simpson.Saffir was commissioned by the United Nations to study low cost housing in hurricane areas. Realizing he needed a scale to gauge the likely effects of hurricanes of differing strengths, he came up with a 1-5 wind speed scale. He passed this on to Robert Simpson at the National Hurricane Center, who also calculated the effects of storm surge and flooding.

The scale only measures these factors and does not take location into account, so storms of lesser category can still be expected to do greater damage over heavily populated areas than on the open sea or in sparsely populated rural areas.

The scale is also only applied to hurricanes that form in the Atlantic Ocean and Northern Pacific Ocean – anything over the International Date Line is called a typhoon or a cyclone and has their own scales.

A storm becomes a hurricane and is graded according to the Saffir/Simpson scale when it exceeds 73 miles per hour.

A category 1 hurricane has sustained wind speeds of 74-95 miles per hour, with a storm surge of 4-5ft. A category 1 hurricane will cause no real damage to building structures, but unanchored mobile homes, shrubbery and trees are at risk. There may also be some coastal flooding and damage to piers.

A category 2 hurricane will sustain winds of 96–110 mph, and a storm surge of 6–8 ft. This will cause some damage to roofing material, doors, and windows. There will be considerable damage to vegetation, mobile homes, and so on. Flooding will likely damage piers, and unprotected small craft in may break their moorings.

A category 3 hurricane has reached 111–130 mph, and has a storm surge of 9–12 ft. At this speed there will be some structural damage to small homes and buildings, mobile homes will be destroyed, and there may be inland flooding. Small structures are at risk from coastal flooding, while larger structures may be hit with floating debris due to coastal flooding.

A category 4 hurricane moves at sustained speeds of 131–155 mph, and has a storm surge of 13–18 ft. The potential for damage is much more severe, with more extensive curtain wall failures and complete roof structure failure on small homes. Beach areas will suffer major erosion, and inland flooding may be more extensive.

A category 5 hurricane is the most feared. Sustaining winds of 156 mph and up, and with a storm surge of 19 ft and more, full scale evacuation of residential areas may be required. The effects will be complete roof failure on many homes and larger buildings. Complete building failures, with small utility buildings blown over or away, may be expected. All structures near the shoreline may sustain major damage from flooding.

Katrina was a category 5 hurricane when it devastated the Gulf Coast in August 2005, killing 1,836 people, with more than 700 still missing, months later. Katrina was the third strongest hurricane ever to make landfall in the US, but one of the costliest and most deadly ever recorded.

All of which begs the question: is there, or should there be, a category 6 for hurricanes? The answer is no on all counts. The categories are 1-5 and do not go over 5. There is no need to have a measure greater than that, according to Robert Simpson, since a category 5 will cause ``serious damage to a building no matter how well it’s (the building) engineered,” and the scale only exists to measure the damage to man made structures.

Nevertheless, some scientists are pushing for a category 6 to take into account the massive and unprecedented loss of life and property that occurred with Katrina.

But for the forseeable future, the scale remains at 1-5 and knowing how a hurricane is measured helps the layman to see how it is recatogorised up and down as it gains and loses speed. It also helps people to understand that evacuation is not an over-reaction by authorities when it is required.

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