For many years Western Hemisphere hurricanes have been categorised according to their wind strength.
The five-point Saffir-Simpson scale has been around since the late 1970s, and has been in its current form, based solely on wind speed, since 2009.
The scale is as flows:
Category 1 119-153km/h
Category 2 154-177km/h
Category 3 178-208km/h
Category 4 209-251km/h
Category 5 above 252km/h
Category 5 hurricanes remain rare events. Since accurate recordings of wind strengths using satellite imagery began in the 1960s, there have only been 33 Category 5 cyclones in the northern Atlantic, Caribbean and Gulf of Mexico.
More powerful cyclones are a direct result of a warming world.
All the evidence suggests that rising temperatures, whilst not increasing hurricane frequency, provide more energy to develop their intensity.
So with hurricanes packing sustained winds more than 30km/h above the Category 5 threshold, is it time to amend the Saffir-Simpson scale to add a Category 6?
This is being discussed at a climate conference in Wellington, New Zealand.
The idea certainly has some high-profile supporters.
Michael Mann of Penn State said: “Scientifically, [six] would be a better description of the strength of 200mph (320km/h) storms, and it would also better communicate the well-established finding now that climate change is making the strongest storms even stronger.”
New Zealand’s climate change minister, James Shaw, was quoted as saying: “The international cyclone rankings don’t go higher than Category Five, the only reason it wasn’t a Category 6 cyclone is because we don’t have a category six, but we might need one in the future.”
Perhaps the final word should go to Robert Simpson, the former director of the US National Hurricane Center, who, along with engineer Herbert Saffir, devised the system.
He felt that as the original scale was related to damage inflicted upon buildings, there comes a point when no building can withstand the force of the wind, rendering higher categories superfluous.
Simpson died in 2014, at the age of 102. But in 2001 he was quoted as saying, “that … when you get up into winds in excess of 155 mph (249km/h) you have enough damage if that extreme wind sustains itself for as much as six seconds on a building it’s going to cause rupturing damages that are serious no matter how well it’s engineered”.