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Recent mega-hurricanes prompt calls for a new Category 6

•, By Michael Irving

Now, scientists at Lawrence Berkeley National Laboratory suggest there's room for a Category 6 on the scale – with five storms in the past decade already reaching that strength.

Currently, the National Hurricane Center uses a measure called the Saffir-Simpson Windscale to classify the intensity of hurricanes in the Western Hemisphere and alert people in the area to take appropriate precautions. It's based on the maximum wind speeds in the storm averaged over a minute – it starts at 74 mph (119 km/h) for a Category 1 hurricane, and goes through different thresholds all the way up to Category 5, which is everything over 157 mph (252 km/h). But because the destructive power of wind increases exponentially, and with hurricane strength growing in recent years, scientists at Berkeley Lab and the First Street Foundation weren't sure if that scale told the full story.

"Our motivation is to reconsider how the open-endedness of the Saffir-Simpson Scale can lead to underestimation of risk, and, in particular, how this underestimation becomes increasingly problematic in a warming world," said Michael Wehner, first author of the study.

Tropical storms form out of interactions between warm ocean water and warm, humid air, and those temperatures are increasing rapidly, thanks to human-induced climate change. This seems set to boost not just the intensity of hurricanes but the speed that they gain power.

In the new study, the team added an extra hypothetical category to the scale. Based on the ranges of lower categories, they suggested that Category 5 would cover storms with wind speeds between 157 and 192 mph (252 and 309 km/h), while the new Category 6 would include any storm that became more powerful than that upper bound.