adiabatic process
A change in temperature, pressure, or volume, involving no transfer of energy to or from another material or system. In an adiabatic process, compression is accompanied by warming, and expansion by cooling. An adiabatic temperature change thus results from a pressure change. The speed at which the temperature of rising air falls with altitude is the adiabatic lapse rate. Dry, rising air expands with height. The energy needed for this expansion comes from the air itself in the form of heat.
The resulting change in temperature is expressed in the equation:
where Dt is the temperature change, g is the acceleration due to gravity, dz is the height change, and Cp is the specific heat of the air parcel. This change is the dry adiabatic lapse rate (DALR): 9.84 °C/1000 m. The temperature change sustained by any parcel of dry air is calculated using Poisson's equation.Dt dz
=-g Cp
If the rising air becomes saturated to dew point, condensation of vapour will begin. This condensation is accompanied by the release of latent heat, which partly offsets the cooling with height, so that the rate of cooling of moist air—the saturated adiabatic lapse rate (SALR)—is lower than the DALR. In the lower troposphere, the vapour content of air is high so the latent heat of condensation is high; SALRs may be as low as 5 °C/1000 m. In the cold, dry, high troposphere, though, there is little vapour ready for condensation, so the SALR may be close to the DALR. Quantitative expressions of the SALR are therefore quite complex. See also sublimation.
Adiabatic changes rarely occur in the stratosphere because this layer experiences very little vertical atmospheric motion.
