Hurricanes, known as 'tropical cyclones', are the most violent storms on Earth.
They are like giant engines that use warm, moist air as fuel.
This is why they form only over warm ocean waters near the equator.
The warm, moist air over the ocean rises upward from near the surface.
As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. The whole system of clouds and wind spins and grows, fed by the ocean's heat and water evaporating from the surface.
As the storm system rotates faster and faster, an eye forms in the centre.
It is very calm and clear in the eye, with very low air pressure. Higher pressure air from above flows down into the eye.
When the winds in the rotating storm reach 39 mph, the storm is called a "tropical storm."
And when the wind speeds reach 74 mph, the storm is officially a "tropical cyclone," or hurricane.
Source NASA