A tropical cyclone occurs when calm, warm seas provide heat and moisture to surface air, causing it to rise into the stratosphere. As the air cools, it deposits moisture and falls, creating a convection current. This cyclical air current begins to rotate, absorbing more energy and becoming a powerful storm.
When warm air rises from the ocean's surface, it creates an area of low pressure. Outside air rushes in to correct the pressure imbalance, creating wind and providing energy to the storm system. The rising air creates clouds and precipitation as moisture condenses at higher altitudes, and the Earth's coriolis effect eventually causes these clouds and the air currents driving them to begin rotating. As more heat and moisture flow upward, more energy flows into the storm system, and wind speeds increase as more air flows in to feed the cyclone. When wind speeds reach 39 miles per hour, the system becomes a tropical storm, and if winds reach 74 miles per hour, it becomes a hurricane.
Tropical cyclones generally form within 300 miles of the equator. This is because the coriolis force is strongest there, allowing the storm to rotate and maintain its strength. Without the rotating convection current, the storm cannot build power and maintain its integrity.