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General Hurricane Facts

Hurricane season is the time that most hurricanes occur. It is from June 1 until November 30.

Hurricanes are on average 340 miles in diameter.

Hurricane winds blow in a counterclockwise spiral around the calm, roughly circular centre called the eye. The eye is the warmest part of the storm.

Surrounding the eye is the eyewall, a wall of thunderclouds. The eyewall has the most rain and the strongest winds of the storm, gusting up to 225 miles per hour in severe hurricanes. The smaller the eye, the stronger the winds.

If you are facing in the direction that the hurricane is traveling, the right side generally has the fastest winds and the left side usually has the most rain.

Air Pressure is the weight of the column of air that extends from the ground (or waterís surface) to the top of the atmosphere. The air pressure is very low in a hurricane.

Hurricane Development

Hurricanes form in the tropics between latitudes 8 degrees and 20 degrees

To form and develop hurricanes must be supplied with a constant supply of warm humid air and this air only exists over oceans with a temperature greater than 80 degrees Fahrenheit. Further, for a hurricane to form this warm surface water must exist in a layer that is at least 600 feet deep.

Hurricanes are actually powered by the (latent) heat energy released from condensation.

Initially a hurricane begins as a group of unorganized thunderstorms. To develop into a hurricane, significant cyclonic circulation must occur around the disturbances. This circulation provides additional moisture and (latent) heat energy and this in turn enhances and intensifies the development of the thunderstorms. The thunderstorms also begin to organise themselves into spiral bands that swirl cyclonically towards the centre of the storm.

A hurricane goes through many stages as it develops. It starts as a tropical wave (typically a westward moving area of low air pressure). As the warm, moist air over the ocean rises in the low air pressure area, cold air from above replaces it. This produces strong gusty winds, heavy rain and thunderclouds that is called a tropical disturbance

If the sustained wind speed around the disturbance increases to between 23 to 38 miles per hour the storm becomes classified as a Tropical Depression. Tropical Depressions can continue to intensify and officially become known as a Tropical Storm at 39 miles per hour. Finally, tropical storms become hurricanes when their sustained wind speed reaches 74 miles per hour.

Hurricanes end (disintegrate) when they travel over land or cold water. Its energy source (warm water) is gone and the storm quickly weakens.

The Eye

Mature hurricanes usually develop a cloud free eye at their centre. In the eye, air is descending creating clear blue skies.

The eye of the hurricane maybe 12 to 31 miles in diameter.

Surrounding the eye are bands of organised thunderstorm clouds and the strongest winds and heaviest precipitation are found in the area next to the eye.

What is it about a hurricane that causes damage?

The damage that hurricanes inflict is usually caused by high winds, heavy rainfall, storm surge, and tornadoes.

High winds cause damage by blowing down objects, creating choppy waves and high seas which can inundate coastal areas with seawater. Wind speed in a hurricane is usually directly related to atmospheric pressure. The lower the pressure the faster the winds blow.

Rainfall within a hurricane can exceed 24 inches in a 24-hour period and if this rainfall occurs on land, flooding often occurs.

Storm surge is an increase in the height of the oceanís surface. It occurs when low atmospheric pressure causes the ocean surface to expand, and because the hurricanes cyclonic winds blow water towards the eye. Some hurricanes have produced storm surge of more than 24 feet.

Tornadoes are known to occur in about 25 percent of the hurricanes that make landfall and they can cause a considerable amount of damage. Some scientists also suspect that the thunderstorms that occur near the eye of a hurricane can produce very strong downbursts (vertical downward movements of air).

Last Updated: 2010-08-15