Hurricanes are caused by the convection currents set up when warm, calm seas heat up the air above the ocean's surface. As the warm, moist air rises, it is replaced by cooler air spiraling down from higher in the atmosphere. This creates a rotating circulation that forms the hurricane.
For a hurricane to spawn, conditions need to be exactly right. The sea has to be relatively calm, and the air above moist and cool at high altitudes. Too much turbulence causes the air circulation to break up before the storm forms, and if the air is too dry, the moisture condenses too quickly and forms small storms. If the conditions suit the formation of a tropical cyclone, the rotating circulation draws more moisture and energy up from the lower atmosphere, building a powerful storm system over time.
Hurricanes grow stronger over warm waters and begin to die when those conditions change. Typically, a hurricane builds in force steadily as it approaches land, but begins to dissipate quickly once the storm leaves the ocean.
Hurricanes generally spawn close to the equator. Storms that affect the Americas form in the calm waters between Africa and South America, and sometimes come together in the Caribbean. Asian storms usually form in the Eastern Pacific before tracking westward. The Indian Ocean also serves as a spawning ground for hurricanes that threaten the Indian subcontinent.