adiabatic lapse rate, rate at which the temperature of an air parcel will rise or fall as it descends or ascends, respectively, without exchanging heat with its environment. The temperature change occurs when the parcel moves into regions of varying barometric pressure and either expands (as pressure falls) or contracts (as pressure rises). A distinction is usually made between the dry and the moist adiabatic lapse rates. The dry rate for a gas such as air depends only on the specific heat of the gas at constant pressure and the acceleration caused by gravity. The dry adiabatic lapse rate for the Earth’s atmosphere equals 9.8° C per kilometre (28.3° F per mile); thus, the temperature of an air parcel that adiabatically ascends or descends 5 kilometres (3 miles) would fall or rise 49° C (85° F), respectively. When an air parcel that is saturated with water vapour ascends adiabatically, some of its moisture will condense, releasing heat and causing the parcel to cool more slowly than it would if it were not saturated. This moist adiabatic lapse rate varies greatly. The greater the amount of moisture contained in the air, the smaller the adiabatic lapse rate; as the air parcel rises, cools, and loses its moisture through condensation, its lapse rate increases and approaches the dry adiabatic.

At the extremely low temperatures of the upper troposphere and of polar regions there is little difference between the two lapse rates. it is usually assumed that when moisture condenses it is lost to a saturated parcel, so that if the parcel descends it warms at the dry adiabatic rate. This concept is often used to explain the warmth of low-lying regions on the lee side of mountain ranges. Moist air ascending on the windward side loses its moisture and cools slowly. As it descends on the lee side, it heats rapidly and reaches temperatures considerably higher than those at the same elevation on the windward side. A careful distinction should be made between the adiabatic lapse rate and the actual, or environmental, lapse rate at a given time. The adiabatic lapse rate is a good approximation to the rate at which the temperature of an air parcel will change as it moves vertically through the free atmosphere, whereas the actual lapse rate, which is established by radiation, convection, and condensation processes, is highly variable and averages about 6.5° C/kilometre (18.8° F/mile).

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