How do tropical cyclones form and intensify ?
Early stages in the development of a tropical cyclone demonstrate nicely the way in which positive feedback causes growth. These steps follow one another – A cluster of thunderstorms (in or near the ITCZ, over warm water) heats the middle and upper troposphere, by latent heat release. This heating only occurs when the air parcel, rising from the surface, is warmer than the environment, in other words when it is buoyant. And this only happens when the environmental lapse rate at some levels exceeds the moist adiabatic lapse rate, i.e. when the atmosphere is conditionally unstable. This heating explains the development of a weak surface low, by hydrostatic balance (Note 1.G). This centre of low pressure at sea level triggers winds, which converge into the low and, if the cluster is at least 5 of latitude from the equator, the winds are deflected by the Coriolis force sufficiently to cause cyclonic rotation. The surface winds promote evaporation and heat transfer from the ocean surface in
