Birth
Tropical cyclones form over warm waters from pre-existing weather disturbances. Some of these disturbances emerge every three or four days from the coast of Africa as "tropical waves," as seen in this animated satellite image.
Tropical cyclones can also form from the trailing ends of cold fronts that move into the tropics and stall, or occasionally from upper–level low pressure circulations that work their way down to the surface.
Ingredients for the development of a tropical cyclone
The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on at least six conditions, some of which are shown in the figure above.
- A pre-existing disturbance providing some initial spin and flow into the system (the horizontal orange arrows)
- Warm (at least 80°F/26.5°C) ocean temperatures to a depth of about 150 ft (~50 m)
- Relatively light winds that do not change much in direction and speed up through the atmosphere (called "low wind shear")
- A location at least 300 miles (~500 km) from the equator. Tropical cyclones require some of the spin provided by the earth's rotation-called the Coriolis force-which is zero on the equator.
- An atmosphere that cools enough with height to allow thunderstorms to develop
- An atmosphere that is moist enough to fuel the thunderstorms
Heat and energy for the storm are gathered by the disturbance through contact with warm ocean waters. The winds near the ocean surface spiral into the disturbance’s low pressure area. The warm ocean waters add moisture and heat to the air, which rises. As the moisture condenses into drops, more heat is released, contributing additional energy to power the storm. Bands of thunderstorms form, and the storm’s cloud tops rise higher into the atmosphere. If the winds at these high levels remain relatively light (little or no wind shear), the storm can remain intact and continue to strengthen.