Hurricane categories can be rated as one to five, depending on their severity. Severity is measured primarily by the speed of the wind accompanying the hurricane, and the amount of swell to the sea as the hurricane is approaching land. The mildest hurricane, and this is almost oxymoronic since no hurricane is exactly mild, is a Category One hurricane. The most severe and most deadly is Category Five. The categorization of hurricanes can change as they move, usually becoming less volatile, until they are finally classed as storms.
Classification and measurement for hurricane categories is fairly specific and is based on the Saffir-Simpson Scale. This is a recent development and is not used throughout the world. It was developed in 1969 by Herbert Saffir and Bob Simpson, and refers only to hurricanes occurring in the Atlantic and Pacific Oceans, and only to those storms east of the International Dateline.
Hurricane categories may be described as follows:
Category One has wind speeds of 74-95 mph (119.1-152.89kph), and a sea swell, also called a storm surge of 4-5 feet (1.22-1.524m). Damage to buildings is minimal, but homes that are not properly attached to foundations can be damaged and trees may be blown down.
Category Two features wind speeds of 96-110 mph (154.5-177.02kph) with the sea rising 6-8 feet (1.83-2.44m). Significant damage may be done to unanchored buildings, and these hurricanes may result in damage to the exteriors of buildings. Doors or windows can suffer the effects of a Category Two, and trees, bushes and piers suffer too. Flooding is likely.
Category Three has a surging sea 9-12 feet (2.74-3.66m) above the normal height and has wind speeds of 111-130mph (178.63-209.21kph). In a Category Three, mobile homes are usually destroyed, and anchored residential homes can be structurally impaired. Even larger trees can be torn down with winds so high, and flooding is very common. Power buildings can sustain significant damage worsening the problem.
Category Four has wind speeds of 131-155mph (210.82-249.44kph) and storm surges of 13-18 feet (3.96-5.49m). Hurricane Katrina was a Category Four. Homes may suffer extensive damage, with doors and windows destroyed, and roofs and whole homes collapsing. Most signs are destroyed, and shrubbery and trees demolished. Extensive flooding creates issues too, often leading to worse problems for people stuck in the area after hurricanes of this type. Water supply can easily become polluted and power plants so damaged that power is unavailable for an extended period.
Category Five wind speeds are over 155 mph (249.45kph), and the sea swells over 18 feet (5.49m) above its normal level. The damage of these hurricanes is extreme, with residences destroyed, flooding tremendous, and virtually always loss of power since power plants typically sustain damaged. This last of the hurricane categories is simply the most severe thing you can think of, an imminent disaster that means people should evacuate to safer areas.
All hurricane categories can spell disaster and people should take seriously any warnings to evacuate. Sometimes in hurricane categories people will list a Category Six. This is fictional, and was never part of the Saffir-Simpson Scale. Anything measuring above Category Four is Category Five. Some fictional works specialize in imagining the wind speed and storm surge of hurricane categories beyond five, but such a classification does not, and hopefully will not ever need to exist.