- Warm, wet air
- Warm water
- An appropriate vertical temperature profile
- A location at least five degrees north or south of Equator
- Low vertical wind shear
Tropical cyclones form in tropical and subtropical areas because warm, wet air is necessary. The Sun heats the ocean, causing some of the water to evaporate. The air near the ocean becomes warm and full of water vapor. The warm air rises, creating an area of low pressure. Cooler area moves into the area of low pressure, warms, and then rises also. As the air rises higher into the atmosphere, it cools, causing the water vapor to condense into clouds. This continued process is ultimately the engine the drives the formation of a tropical cyclone.
The water must be at least 26.5°C (80°F) for a depth of about 50 m (150 ft, although meteorologists still don't know an exact depth), and air in the tropics isn't always wet. There must be enough water vapor in the air to allow the cycle of evaporation and condensation to continue. Lastly, the temperatures higher in the atmosphere must be cool enough to allow the rising air to get cool enough to condense. Meteorologists call this an appropriate "vertical temperature profile," meaning the air near the water is warm enough to give the tropical cyclone the energy it needs while higher layers of the atmosphere are cool enough for continual condensation.
Although the ocean is the hottest at the Equator, tropical cyclones do not form there. The force of the Coriolis effect, which causes wind to curve, contributes to the rotation of winds in a tropical cyclone, but at the Equator the force of the Coriolis effect is negligible. This is due to the Earth's spherical shape. A point on the Equator (the widest part of the Earth) has more distance to travel than a point near the pole (the point at the pole is not travel at all; it is the axis around which the Earth is moving). Therefore a point on the Equator has to move faster to match the rotation speed of a point near the pole. This is what causes the Coriolis Effect--points on the Equator are moving faster than all the other points north or south of it, so that if something is moving from the Equator to the north or south, it gets dragged backward. Tropical cyclones usually form at least 500 km (311 mi) from the Equator.
Tropical cyclones do not spontaneously generate. They evolve from a group of thunderstorms, which go through the stages of tropical disturbance, depression and storm before becoming a full-blown hurricane, typhoon or cyclone. Other weather systems can break up this evolution, a prime example being high vertical wind shear. Wind shear, also known as wind gradient, is the change of wind in speed and direction over a short distance. High vertical wind shear is wind that changes speed dramatically with height. Such a wind would basically topple the rotation of a forming cyclone and destroy it.
Meteorologists do not yet know all the conditions that must be met for a tropical cyclone to form, as there have been systems that meet the conditions listed above that did not achieve the status of cyclones.
