A hurricane is an intense weather system in which the maximum average wind speed near a center or eye exceeds 74 mph or 119 km/h. The winds rotate in a counter-clockwise spiral around a region of low pressure. Hurricanes occur in the Atlantic and Eastern Pacific. A hurricane watch is issued when there is a threat of hurricane conditions within 24-36 hours. A hurricane warning is issued when hurricane conditions (winds of 74 miles per hour or greater or dangerously high water and rough seas) are expected in 24 hours or less.
Hurricanes only form over really warm ocean water of 80°F or warmer. The atmosphere (the air) must cool off very quickly the higher you go. Also, the wind must be blowing in the same direction and at the same speed to force air upward from the ocean surface. Winds flow outward above the storm allowing the air below to rise. Hurricanes typically form between 5 to 15 degrees latitude north and south of the equator. The Coriolis Force is needed to create the spin in the hurricane and it becomes too weak near the equator, so it is unlikely hurricanes will form there. (Coriolis Force- A force that deflects moving objects to one side because of the Earth’s rotation. The object is still going straight but the Earth moves underneath it, making it look like it is moving to one side. In the Northern Hemisphere, the Coriolis Force deflects objects to the right).
|Tropical Wave||A low pressure trough moving generally westward with the trade winds.|
|Tropical Disturbance||An organized area of thunderstorms that usually form in the tropics. Typically, they maintain their identity for 24 hours and are accompanied by heavy rains and gusty winds.|
|Tropical Cyclone||A generic term for any organized low pressure that develops over tropical and sometimes sub-tropical waters. Tropical depressions, tropical storms, and hurricanes are all examples of tropical cyclones.|
|Tropical Depression||An organized area of low pressure in which sustained winds are 38 mph or less.|
|Tropical Storm||A tropical cyclone with maximum sustained wind speeds that range from 39 to 73 mph.|
|Hurricane||A tropical cyclone with sustained winds of at least 74 mph.|
Hurricanes begin as tropical storms over the warm moist waters of the Atlantic and Pacific Oceans near the equator. (Near the Philippines and the China Sea, hurricanes are called typhoons.) As the moisture evaporates it rises until enormous amounts of heated moist air are twisted high in the atmosphere. The winds begin to circle counterclockwise north of the equator or clockwise south of the equator. The relatively peaceful center of the hurricane is called the eye. Around this center winds move at speeds between 74 and 200 miles per hour. As long as the hurricane remains over waters of 79 degrees Fahrenheit or warmer, it continues to pull moisture from the surface and grow in size and force. When a hurricane crosses land or cooler waters, it loses its source of power, and its wind gradually slow until it is no longer of hurricane force–less than 74 miles per hour.
Tropical cyclones out at sea cause large waves, heavy rain, and high winds. This can result in the disruption of international shipping and shipwrecks. On land, strong winds can damage or destroy vehicles, buildings, bridges, and other outside objects, turning loose debris into deadly flying projectiles. The storm surge or the increase in sea level due to the cyclone, is typically the worst effect from land-falling tropical cyclones. Historically, storm surges are responsible for 90% of tropical cyclone deaths.
Over the past two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million people worldwide. Large areas of water caused by flooding, including stagnant water, can lead to infection, as well as contribute to mosquito-borne illnesses. Crowded evacuees in shelters increase the risk of disease spreading.
Tropical cyclones significantly interrupt infrastructure, leading to power outages, bridge destruction, and the hampering of reconstruction efforts. On the other hand, these storms sometimes bring much needed rain to very dry areas.