Transition height or cluster ion cut-off level, a characteristic parameter of the ionospheric D-region, can be computed from a knowledge of ion composition data from rocket experiments or by using an empirical formula proposed to be included in the IRI lower ionosphere model. A large number of available normal D-region electron density profiles (from rocket experiments) for low and middle latitudes have been used together with the empirical formula to examine the seasonal variation of transition height, and an inverse dependence on temperature at 85 km has been observed. The seasonal variation of transition height from ion composition data for high latitudes, although shows the inverse relation with the temperature, exhibits characteristics which are different from those observed at low and middle latitudes
