According to the National Oceanographic and Atmospheric Administration, tropical storms form when warm, calm ocean waters create a convection current in the air above the surface. As the warm, moist air rises, it creates rotating clouds, forming the central structure of a tropical storm.
When warm air rises, it creates a zone of low pressure near the surface, drawing in winds from outside the storm's center. As the air reaches cooler parts of the atmosphere, the water condenses out into clouds, and the air begins to sink again at the center of the depression. This creates the circular currents that give the storm its rotation. As long as the system remains over warm, calm waters, it will continue to grow in strength.