Tropical cyclone development criteria

Necessary conditions for the development of tropical storms and hurricanes (from observations) (Gray, 1979):

• The presence of a pre-existing region of lower tropospheric horizontal wind convergence (low-level relative vorticity).

• A location poleward of about 4 - 5^o of latitude.

• An underlying ocean with surface temperatures above 26^oC to a depth of 60m.

• A small vertical shear between the lower and the upper troposphere.

• A distribution of temperature with height that favours the formation of cumoulonimbus clouds.