Hurricanes need several ingredients to form and grow, including warm ocean water, lots of moisture in the air, low vertical wind shear, and thunderstorms (or an area of low pressure). To start, the temperature of the water at the ocean surface needs to be at least 80°F. Then, as warm, moist air rises from the ocean surface, it cools and condenses, creating clouds. With enough moisture these ingredients can develop into towering rainclouds and thunderstorms.
Vertical wind shear occurs when winds higher up are blowing at different speeds and from different directions than winds at the surface of the ocean. When this happens the hurricane can't develop the tall rain clouds needed to sustain the storm. Thus when there is low vertical wind shear, the towering rainclouds can grow and begin to rotate around a central low pressure area (eventually becoming what is known as "the eye"), creating and sustaining a hurricane.
Did you know, that even a medium sized hurricane is capable of producing as much energy as 200 times the world-wide electrical generating capacity?!
Where does all this energy come from?
As the tropical oceans heat up they store vast quantities of energy. When water evaporates off the ocean surface (turns from a liquid to a gas) it creates warm, moist air that stores some of this energy. The warm, moist air then rises, cools, and condenses (turns back to liquid) forming rain clouds and releasing the stored energy in the process. As long as there is enough warm water, this process can continue to build up energy in the rainclouds, eventually creating a hurricane.