Atmosphere And Weather

The Life Cycle of a Hurricane



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Pre-existing weather disturbances, warm tropical oceans, moisture, and light winds. Oceanic and atmospheric conditions play a key role in the life cycle of a hurricane. From tropical depression to hurricane, these tropical cyclones can have a life span as long as two to three weeks.

The birth of a hurricane, or tropical cyclone, occurs over warm waters from pre-existing disturbances. Initially a cluster of thunderstorms over tropical waters, as heat and energy gather through contact with warm waters and winds near the ocean's surface spiral into the low pressure area, these disturbances can form a tropical depression. As an organized system of clouds and thunderstorms, this tropical depression has a defined surface of circulation and maximum sustained winds of up to 38 mph. Once a tropical depression, it takes on the familiar spiral appearance due to wind flow and the rotation of the earth. If the right conditions exist the tropical depression can continue to strengthen into a tropical storm. The amount of time it takes to go from tropical depression to tropical storm can take as little as half a day to a couple days depending on conditions. If conditions deteriorate, this transition may not occur at all.

A tropical storm evolves once a tropical depression has intensified to winds between 39 to 73 mph. At this point it is assigned a name from the predetermined alpha phonetic list for that year. Once it reaches this stage of tropical storm, the bands of thunderstorms contribute additional heat and moisture to the storm. The storm becomes even more organized and circular, resembling the familiar hurricane shape. Just like the transformation from tropical depression to tropical storm, it can take as little as a day to a couple of day for the tropical storm to transition to a hurricane.

Once the storm reaches a minimum wind speed of 74 mph, it is upgraded to a hurricane. It is during this stage that the distinguished "eye" forms at the center of the storm. The hurricane is the most severe category of a tropical cyclone, with winds capable of exceeding over 155 mph. Hurricanes are ranked into categories defining the strength of the storm. They range from Category 1 with winds between 74 to 95 mph to a Category 5 with winds of 155 mph or more. The category of a hurricane however does not directly relate to the damage it will inflict. Hurricanes typically measure about 300 miles in width although the size can vary considerably, hurricane Andrew in 1992 for example was 100 miles wide while Gilbert in 1988 stretched over 500 miles.

The death of a hurricane occurs when the tropical cyclone can no longer extract the sufficient energy it needs to survive from the warm ocean waters. There are many reasons a hurricane may begin to dissipate. Moving over cooler water or drier areas can lead to weakening of the storm, wind shear can also tear apart the hurricane. As a hurricane moves over land, the main moisture source shuts off and the surface circulation gets reduced by friction. A weakening storm can re-intensify if it moves into more favorable conditions.

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