How Do Tropical Cyclones Form?
For a cyclone to form several preconditions must be met: Warm ocean waters (of at least 26.5°C) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m). Warm waters are necessary to fuel the heat engine of the tropical cyclone. An atmosphere which cools fast enough with height (is "unstable" enough) such that it encourages thunderstorm activity. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development. Relatively moist layers near the mid-troposphere (5 km). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity. A minimum distance of around 500 km from the equator. Some of the earth's spin (Coriolis force) is needed to maintain the low pressure of the system. (Systems can form closer to the equator but it's a rare event) A pre-existing disturbance near the surface with sufficient spin (vorticity) and inflow (convergence). Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organised system with sizeable spin and low level inflow. Little change in the wind with height (low vertical wind shear, i.e. less than 40 km/h from surface to tropopause). Large values of wind shear tend to disrupt the organisation of the thunderstorms that are important to the inner part of a cyclone. Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favourable conditions do not develop.
