1. The state of a column of air in the atmosphere when it has a superadiabatic lapse rate of temperature (i.e., greater than the dry-adiabatic lapse rate). An air parcel displaced vertically would be accelerated in the direction of the displacement. The kinetic energy of the parcel would consequently increase with increasing distance from its level of origin. See parcel method, conditional instability, absolute stability. 2. (Also called mechanical instability. ) The state of a column of air in the atmosphere when its lapse rate of temperature is greater than the autoconvective lapse rate. In such a column the air density would increase with elevation. See also autoconvection.