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Many Names for One Storm Type
The hurricane type of tropical cyclone occurs around the world, but different names are given to the storms depending on where they occur. Hurricane applies to tropical cyclones of the Western Atlantic and Eastern Pacific regions. In the Indian Ocean and South Pacific, they retain the name cyclone. In the Western Pacific, tropical cyclones are called typhoons. Meteorologists detect and track forming cyclones, issuing warnings when they reach hurricane strength. Hurricanes occur most frequently from June 1 through Nov. 30 in the Atlantic, Caribbean and Gulf of Mexico and from May 15 through Nov. 30 in the eastern Pacific, but tropical cyclones can form year-round.
Hurricane Classification
Tropical cyclones that qualify as hurricanes are categorized into five different intensities based on the maximum sustained surface wind peak at the height of 10 meters (33 feet) from the ground. Called the Saffir/Simpson Hurricane Wind Scale, there are modified versions for different geographical locations. Wind speeds start at between 64 to 82 knots (74 to 95 miles per hour or 119 to 153 kilometers per mile) in Category 1 storms, extending to Category 5 with sustained winds greater than 136 knots (156 miles per hour or 251 kilometers per hour). Major hurricanes are those that reach Category 3 status.
Birth of Cyclonic Storms
To make a cyclone, ocean waters need a surface temperature of 26.5 degrees Celsius (80 degrees Fahrenheit) or more. Specific conditions have to be present before a cyclone forms. The warm ocean waters need to extend to at least 50 meters (150 feet), and thunderstorm activity is needed to liberate the ocean's heat energy to fuel the cyclone. Favorable locations for cyclone formation exist a minimum distance of 500 kilometers (300 miles) from each side of the equator, where the Coriolis forces help maintain the low pressure area. Wind speeds can't change much with height, since that interferes with formation of the cyclonic wind pattern.
Cyclonic Storm Stages
A developing tropical cyclone goes through several stages before it becomes a hurricane. At Stage 1, a thunderstorm that remains intact for 24 hours or more is called a tropical disturbance. When the disturbance acquires a closed circulation with winds blowing around the center of low pressure, it is a Stage 2 tropical depression. Winds rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The cyclone goes to Stage 3, a tropical storm, when one-minute wind speeds are between 34 to 63 knots (39 to 73 miles per hour or 63 to 117 kilometers per hour) and is given a name. The final Stage 4 is the hurricane status.