Hurricanes are powerful tropical cyclones that can last for weeks and devastate large areas with powerful winds and flooding. Unlike tornadoes, which can form quickly and with little warning, hurricanes require a very specific set of conditions and take some time to develop. Forecasters watch carefully for these conditions in order to predict these dangerous storms.
The most important ingredient in the formation of a hurricane is warm, humid air, which is why most form in the region close to the equator. Hot, humid air over the ocean rises, reducing the pressure below it. As the air rises and cools, it forms clouds. When more air flows up into the system, the cooler, cloud-laden air begins to move, starting the rotation of the storm. The Coriolis effect created by the Earth’s rotation causes storms in the Northern Hemisphere to rotate counterclockwise, while cyclones in the southern half of the world spin the other way.
The first stage of a hurricane is the “tropical depression” stage. For a storm to be classified as a tropical depression, it needs to be a low-pressure system involving thunderstorms, with wind speeds up to 61 kilometers per hour (38 mph or 33 knots). At this point, the beginnings of rotation occur, but the storm remains disorganized and does not present a clearly formed eye. Some tropical depressions collapse, while others move through the ocean, gathering strength and increasing in intensity. The National Hurricane Center does not name tropical depressions but assigns each system a number.
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If a tropical depression strengthens sufficiently, it becomes a tropical storm. Tropical storms have winds ranging from 63 to 117 kilometers per hour (39 to 73 mph or 34 to 63 knots) with organized rotation. At this point, dense rain bands form, and the storm system may be hundreds of miles across. At this stage of development, the NHC provides the storm system with a name from a pregenerated alphabetical list, and the system will bear that name until it dissipates.
Once a tropical storm generates sustained winds above 119 kilometers per hour (74 mph or 64 knots), it becomes a Category 1 hurricane on the Saffir-Simpson Hurricane Scale. These storms present powerful rain bands, well-defined rotation and a central eye, a calm spot in the center of the storm. If the storm reaches 179 kilometers per hour (111 mph or 96 knots), or a Category 3 storm, the NHC classifies it as a major hurricane. The most powerful storms reach Category 5, with sustained winds over 249 kilometers per hour (155 mph or 135 knots). Hurricanes begin to lose intensity once they make landfall, or when they encounter certain meteorological conditions, and the National Weather Service will continue to track and monitor a system until it passes below tropical depression strength and dissipates.