As it named, tropical cyclone formed within tropical area, particularly above warm ocean.
Averaged number of tropical cyclone developed in northern hemisphere is annually about 57.3 and 26.3 in southern hemisphere (based on data from 1968 to 1989).
Tropical cyclone development requires this following condition:
- Sea surface temperature at least 26.5° C to 60 meters depth.
- Unstable atmosphere with possibility to Cumulonimbus cloud development. Cumulonimbus cloud, formed in deep convective environtment, is important in the development of cyclone.
- Humid atmosphere in mid level atmosphere (about 5 kilometers altitude). Dry condition at this level is not favourable for development of thunderstorm activity inside the cyclone.
- About 500 kms away from the equator. Though possible, it is rare for tropical cyclone formed near the equator.
- Atmosphere disturbance near earth surface
- Little variation of wind with height. High changes of wind direction or speed with height will dessipate the thunderstorm.