A hurricane is a powerful, swirling storm system that begins over a warm sea. Hurricanes form in waters near the equator, and then they move toward the poles. The winds of a hurricane swirl around a calm central zone called the eye surrounded by a band of tall, dark clouds called the eyewall.
Hurricanes are referred to by different labels, depending on where they occur. They are called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons if they occur in the Northwest Pacific Ocean. Near Australia and in the Indian Ocean, they are referred to as tropical cyclones.
Hurricanes are most common during the summer and early fall. Typhoons occur throughout the year in the Northwest Pacific but are most frequent in summer. Approximately 85 hurricanes, typhoons, and tropical cyclones occur in a year throughout the world.
A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 39 miles per hour. It has no eye and does not typically have the organization or the spiral shape of more powerful storms.
A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 39 miles per hour and 73 miles per hour. At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Viewed from above, the storm clouds now have a well-defined circular shape. The strong winds near the surface of the ocean draw more and more heat and water vapor from the sea. The increased warmth and moisture in the air feed the storm.
Each tropical storm receives a name. The names help meteorologists and disaster planners avoid confusion and quickly convey information about the behavior of a storm.
A storm achieves hurricane status when its winds exceed 74 miles per hour. By the time a storm reaches hurricane intensity, it usually has a well-developed eye at its center. Surface pressure drops to its lowest in the eye. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, in which the strongest thunderstorms and winds circulate around the storm's center.
In the eyewall, warm air spirals upward, creating the hurricane's strongest winds. The speed of the winds in the eyewall is related to the diameter of the eye. If the eye widens, the winds decrease. Heavy rains fall from the eyewall and bands of dense clouds that swirl around the eyewall.
Hurricanes last an average of 3 to 14 days. A long-lived storm may wander 3,000 to 4,000 miles, typically moving over the sea at speeds of 10 to 20 miles per hour. All hurricanes eventually move toward higher latitudes where there is colder air, less moisture, and greater wind shears. These conditions cause the storm to weaken and die out. The end comes quickly if a hurricane moves over land, because it no longer receives heat energy and moisture from warm tropical water. Heavy rains may continue, however, even after the winds have diminished.