According to the National Oceanographic and Atmospheric Administration, hurricanes form when heat from calm, warm seas rises into a cooling atmosphere, generating a tropical system. The rising moisture causes the forming clouds to begin a rotation, much like water flowing down a drain. As the storm builds, it draws more and more energy from the ocean's warmth until it reaches hurricane strength.
When the air nearest the surface of the ocean is considerably warmer than the air above, it begins to rise. As that air moves upward, it creates an area of low pressure near the surface, and nearby higher-pressure air moves in to fill this void. This air movement is the engine that initiates the rotation of the tropical depression.
As the storm builds, the winds associated with it grow stronger and stronger, changing the classification of the storm. Any tropical system with winds between 25 and 38 miles per hour is a tropical depression, while winds of 39 to 73 miles per hour indicate a tropical storm. Once wind speeds top 74 miles per hour, the storm officially becomes a hurricane, with five levels of severity depending on its wind speed. Generally, the longer a storm spends over warm ocean waters, the stronger it will become.