Adiabatic Processes

Definition

A process affecting a parcel of matter is said to be adiabatic if it occurs without addition or loss of heat from the parcel. No exchange of heat means that temperature changes in the parcel must be due to changes in pressure alone. In planetary atmospheres, the adiabatic lapse rate is the change in air temperature with changing height, resulting from pressure change. The so-called dry adiabatic lapse rate has the slope $\mathrm{d}\left( \ln \kern0.5em \mathrm{T}\right)/\mathrm{d}\left( \ln \kern0.5em \mathrm{p}\right)=\mathrm{R}/{\mathrm{c}}_{\mathrm{p}}$, where T is temperature, p is pressure, R is the specific gas constant (which depends on the mean molecular weight of the mixture), and c_p is the specific heat at constant pressure. For Earth’s atmosphere, the value R/c_pcorresponds to about 2/7. Adiabatic processes are called isentropic, i.e., they leave entropy unchanged, provided that the changes in state of the system are slow enough that the system remains close...