As hurricane season ramps up, what is a Category 5? Why is there no Category 6? Hurricanes are measured on the Saffir-Simpson scale, which was first developed in 1971. It was developed by civil engineer Herbert Saffir and meteorologist Robert Simpson who was running the National Hurricane Center (NHC) at the time. It was introduced to the public in 1973. The scale at first incorporated other factors, but now refers simply to how fast the wind is blowing. A Category 5 hurricane means that it has winds blowing 157 miles per hour or more.
I suppose we've all been watching Hurricane Irma rip through the Caribbean like a giant buzzsaw blade, with wind speeds over 180mph. This is one of those rare Category 5 storms. But just how rare are Category 5 hurricanes? According to the Wikipedia page, hurricanes with wind speeds 157mph are considered Category 5. So let's dig up some data, and see how many hurricanes we've had in the Atlantic with those kind of wind speeds ... After a bit of digging, I found a good repository of data about past hurricanes on the weather.unisys.com
Another unprecedented event is taking place in this year of unusual tropical cyclones. Hurricane Matthew is heading for Jamaica, from the south. It has already taken an unusual track; coming from Martinique as a Tropical Storm, Matthew headed west-southwestwards towards Colombia, strengthening to category 3 as defined by the Saffir-Simpson Scale. On Friday night the hurricane was within 120km of Punta Gallinas, the most northerly point of South America. This is as close as any hurricane has been recorded as coming to the continent.