Instability that is caused by the rising of very dry air over warm, moist air below. C.-G. Rossby (1932) argued that it exists when one of these conditions is met over a layer of atmosphere: the lapse rate of wet-bulb temperature exceeds the moist-adiabatic lapse rate; the equivalent potential temperature decreases with height; the wet-bulb potential temperature decreases with height. Convective instability of the terrestrial atmosphere is largely determined by water vapour. See Gutzler (1996) J. Atmos Scis 53, 19.