Hurricanefacts Tropical cyclones are called hurricanes in the Atlantic and typhoons in the Pacific. Hurricanes rotate anti-clockwise in the northern hemisphere, and clockwise in the southern hemisphere. For a hurricane to form, ocean temperature must be at least 26½°C down to a depth of at least 50m. One third of Atlantic hurricanes form in September, and three quarters between August and October.
Hurricane force: Force 1 (minimal): 74-95mph [110 have hit the US since 1851, 1 of them this year] Force 2 (moderate): 96-110mph [72 have hit the US since 1851] Force 3 (extensive): 111-130mph [73 have hit the US since 1851, 2 of them this year] Force 4 (extreme): 131-155mph [19 have hit the US since 1851, 1 of them this year] Force 5 (catastrophic): 156+mph [3 have hit the US since 1851 - in 1935, 1969 and 1992]
Hurricane Able was the first named hurricane, in 1950. Only female names were used from 1953 until 1979 - now hurricanes alternate in gender. A list of 21 names is picked for each hurricane season. Lists repeat every 6 years. This year's list will be reused in 2011, but Katrina and Rita will almost certainly be retired. There are only four names left on this year's list. If the names run out (which would be the first time ever) then Greek letters will be used instead.