Today, a hurricane is defined as a tropical cyclone with sustained winds that have reached speeds of 74 mph or higher. These storms reach the status of "hurricane" only after strengthening over a period of days or even weeks.
This process begins in the warm, moist air over the waters of the region known as the tropics, which includes the Gulf of Mexico, the Caribbean Sea, the eastern North Pacific Ocean and the North Atlantic Ocean, east of the International Dateline and north of the equator. Tropical depressions, tropical storms, hurricanes, and typhoons are all tropical cyclones, and all may develop in this region.
Before it can become a hurricane, a tropical cyclone -- which is a low pressure system with a defined wind circulation that develops over the tropics -- it must pass through four distinct stages: tropical disturbance, tropical depression, tropical storm and finally, hurricane.
Stage 1: Tropical disturbance
A tropical disturbance is a discrete system of clouds, showers, and thunderstorms that originates in the tropics and remains intact for 24 hours or more.
Tropical waves are a type of tropical disturbance that develops about every four to five days, and some of these waves eventually strengthen to become tropical storms and hurricanes. Sometimes called easterly waves, they are
areas of low pressure that move generally from east to west, embedded in the tropical easterly winds.
Stage 2: Tropical depression
When a tropical disturbance develops a closed circulation (e.g., counter-clockwise winds blowing around a center of low pressure in the Northern Hemisphere), it is designated as a tropical depression. Tropical depressions contain maximum sustained one-minute winds of 38 mph (33 knots) or less, at an elevation of 10 meters.
Stage 3: Tropical storm
A tropical cyclone is given a name by the National Hurricane Center once it reaches tropical storm status. Tropical storms have maximum sustained one-minute winds of 39-73 mph (34-63 knots), at an elevation of 10 meters.
Stage 4: Hurricane
Hurricanes have sustained one-minute winds of at least 74 mph (64 knots), at an elevation of 10 meters.
Winds in most hurricanes can become much stronger.
Hurricanes are categorized on a scale of 1 to 5 based on their wind speed, a scale known as the Saffir-Simpson hurricane wind scale, named after its originators, Herbert Saffir and Dr. Robert Simpson.
While this satellite photo of a hurricane features a tightly formed and easily visible eye at its center, not all hurricanes appear this way. See more photos of hurricanes. In developing tropical cyclones, strong thunderstorms occur. Air pressure drops at the surface of these storms. This low pressure attracts warm moist air from the ocean's surface. The Coriolis force causes the resulting low-level winds to spiral in a counterclockwise direction around the center of the low in the Northern Hemisphere. (Winds swirl clockwise in the Southern Hemisphere.)
Typically, an "eye" forms when the tropical cyclone reaches hurricane strength, but an eye is not necessary for a tropical cyclone to become a hurricane.
Another way to think of a hurricane is as a large heat engine. The fuel is moisture from warm ocean water. The moisture is converted to heat in the thunderstorms that form. Spiral rain bands that surround the tropical cyclone's core help feed the circulation more heat energy.
As air nears the center, it rises rapidly and condenses into clouds and rain. The condensation releases tremendous amounts of heat into the atmosphere. The result is lower surface pressure and strengthening winds.
In this way, the tropical cyclone's engine refuels itself, concentrating its power in a donut-shaped area, called the eye wall, surrounding the center. The eye wall typically contains the strongest surface winds.
Sinking air at the center clears the tropical cyclone of clouds and forms the "eye." Falling surface pressure can occur only if air mass is removed from the circulation center. This is accomplished by wind flowing away from the circulation in the upper atmosphere.