Hurricanes result from an increasingly strong storm system over water.
When air over warm ocean water is heated by the Sun to about 81° Fahrenheit (27° Celsius), it rises in severely increased updrafts and low pressure.
Numerous, large convection cells (circulation patterns of warm air rising, cooling, and condensing) go to work creating a widespread storm, lowering the air pressure more and more. The cells merge and great winds begin to blow as surrounding high pressure air moves in to equalize the low pressure.
Winds tend to blow in the same direction, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, which creates a whirlpool, or cyclone.
Meanwhile, the convection cell is constantly fed by the heat of condensing air and the wind, growing larger and stronger.
When the winds reach a speed of about 75 miles (120 kilometers) per hour, the storm has become a hurricane.
Hurricanes can grow to be over 1,000 miles (1,600 kilometers) wide with wind speeds of 200 miles (320 kilometers) per hour.