The rate of decrease of the temperature of an air parcel as it is lifted, −dT/dz, or occasionally dT/dp, where p is pressure. The concept may be applied to other atmospheric variables, for example, the process lapse rate of density. The process lapse rate is determined by the character of the fluid processes and should be carefully distinguished from the environmental lapse rate, which is determined by the distribution of temperature in space. In the atmosphere the process lapse rate is usually assumed to be either the dry-adiabatic lapse rate or the moist-adiabatic lapse rate.