A lapse rate over the 9.8 °C/1 000 m dry adiabatic lapse rate, rare in the free atmosphere, but common just above land surfaces emitting strong terrestrial radiation. When the environmental lapse rate is superadiabatic, the density of the atmospheric parcel relative to its surroundings results in a buoyancy force in the same direction as original vertical movement (Slonaker (1996) Weather & Forecast. 11).