Hurricanes start when warm, moist air from the ocean surface begins to rise rapidly, where it encounters cooler air that causes the warm water vapor to condense and to form storm clouds and drops of rain.
Hurricanes begin as tropical storms over the warm moist waters of the Atlantic and Pacific Oceans near the equator. 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 reatively 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 79F 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 they are no longer of hurricane force--less than 74 miles per hour.
Hurricanes over the Atlantic often begin near Africa, drift west on the Trade Winds, and veer north as they meet the prevalling winds coming eastward across North America. Hurricanes over the Eastern Pacific begin in the warm waters off the Central American and Mexican coasts. Eastern and Central Pacific storms are called "hurricanes." Storms to the west of the International Date Line are called "typhoons."
Because of the destructive force of hurricanes during late summer and early autumn, scientists constantly monitor them with satellites and sometimes even fly airplane surveillance to keep track of tropical storms that might develop into hurricanes.
