Hurricane Season

The Atlantic tropical cyclone season (including the Gulf of Mexico and the Caribbean Sea) typically peaks during late summer and early fall. This is generally the time that the ocean's water temperature is the warmest. Hurricane season runs from June 1 to November 30.

Tropical Cyclones

The word hurricane is derived from the term urican or unrican used by the ancient Carib Indians to describe the big autumn storms that plagued the Caribbean Sea. Hurricanes are called typhoons in the western North Pacific Ocean, and cyclones in the Indian Ocean and off the coast of Australia.

The terms "typhoon," "cyclone," and "hurricane" are regionally specific names for a strong "tropical cyclone." A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation. Atlantic tropical cyclones form off the coast of western Africa, over the Caribbean Sea, or over the Gulf of Mexico and generally track west or north.

Formation

Tropical cyclones get their start along the equatorial trough or inter-tropical convergence zone. The warm ocean, high humidity, and colliding hemispheric winds trigger the formation of low-pressure systems. Water vapor, which comes from the ocean surface, rises high into the sky. The rising warm moist air, "convection," produces clouds and rain as it cools and condenses in the higher altitudes. The lower atmospheric pressure caused by the intense humidity and colliding hemispheric winds trigger the formation of low-pressure systems. The lower atmospheric pressure caused by the intense convection starts to spin up into a circulation. As warm air continues to rise and produce latent heat which fuels the developing low, the atmospheric pressure continues to fall. The falling pressure forces the surrounding air to rush in toward the center of lowest pressure. The "Coriolis Force," caused by the rotation of the earth, forces the moving air to bend to the right in the northern hemisphere. The air then spins around the low center in a counterclockwise motion and accelerates as the pressure falls. The lower the pressure is, the faster the air moves around it.

Tropical cyclones can also develop from easterly waves or troughs, which originate over Africa in the Sahara Desert. These small westward moving disturbances or waves in the tropics often produce fair weather and northeast winds in advance of the trough then southeast winds and rain squalls behind the trough. If enough rotation is available, an easterly wave may develop a closed circulation and eventually develop into a tropical cyclone. The Cape Verde-type hurricanes are Atlantic basin tropical cyclones that also move off of Africa and frequently develop into tropical cyclones near the Cape Verde Islands and then become hurricanes before reaching the Caribbean.

The main weather patterns in the upper levels of the atmosphere then push the developing storm across the Atlantic Ocean. If all atmospheric conditions are favorable for cyclone development, the system will likely reach hurricane intensity. Once the storm encounters either strong upper level wind, colder air or ocean conditions, or moves over land, it will begin to dissipate.

Nature and Structure

In appearance, a tropical cyclone resembles a huge whirlpool - a gigantic mass of revolving moist air. Most of the heavy rain occurs near the storm center and along spiral rain bands. The rain bands rotate in the same sense as the storm circulation and tend to sweep through an area one after another. At a given location, heavy precipitation is usually pulsing at intervals of a few hours. Squalls and gusts increase during the approach and passage of rain bands. Rain becomes persistent and winds violent as the center of the storm draws near.

Size

Hurricanes come in all sizes. Some extend 1,000 miles across while other midget storms cover only 100 miles or less. The gale force wind radius in a storm usually covers an area of 500 miles in an average size storm. The hurricane strength winds usually cover an area of 100 miles across in average size hurricanes.

Eye

The highest winds are right around the calm eye in the eye-wall of the hurricane. The winds slowly decrease in strength as they move out and away from the eye-wall.

Wind Direction

Always remember a hurricane wind field rotates counterclockwise around the center or calm eye. Consequently, you can always know where the low-pressure center is in relation to Key West. Buys/Ballots Law of Storms allows you to always know where an approaching storm is. If you face the wind, the low-pressure center or eye will always be straight out of your right side. As you follow the wind shift, you can tell where the eye is passing in relation to the land area.

If the wind backs or shifts from east to northeast to north to northwest, you know the eye is passing north of Key West. If the wind veers or shifts from northeast to east to southeast, you know the storm is passing south of Key West. If the winds remain steady from the same direction with no shift, the storm is still heading straight for Key West.

Pressure

Some wrist or dive watches come with a built-in barometer. These are handy for calculating the wind speed as the storm gets closer. It will also tell you whether the storm is coming or going.

If the pressure is falling, the storm is still getting closer to you. If the pressure starts to rise, the storm is moving away. When the pressure remains steady and the wind remains out of the same direction, the storm has probably stalled.

Effects

The destructive effects from a hurricane vary with the cyclone's intensity and size, as well as the location impacted relative to the storm's center. Intense winds, increased sea level, high waves, and torrential rains can be expected. Winds are characteristically stronger on the right side of the cyclone's track.

Storm Surge

Most storm surge is caused by winds pushing the ocean surface ahead of the storm on the right side of the track (left side of the track in the Southern Hemisphere). Individual storm surges are dependent upon the coastal topography, angle of incidence of landfall, speed of tropical cyclone motion as well as the wind strength.

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