LAPSE RATE

rate of temperature change observed in passing upward through the Earth's atmosphere. The lapse rate is considered positive when the temperature decreases with elevation, zero when the temperature is constant with elevation, and negative when the temperature increases with elevation (temperature inversion ). The lapse rate of nonrising aircommonly referred to as normal temperature, or actual, lapse rateis highly variable, being affected by radiation, convection, and condensation processes; it averages about 6.5 C per km (18.8 F per mile). It differs from adiabatic lapse rate, which involves temperature change due to the rising or sinking of an air parcel. Adiabatic lapse rate is usually differentiated as dry or moist. The dry rate for air depends only on the specific heat of air 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 ascends or descends 5 km (3 miles) would fall or rise 49 C (85 F), respectively. When an air parcel that is saturated with water vapour rises, 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 considerably. 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. The comparison between the normal lapse rate in the atmosphere and the dry and moist adiabatic lapse rates determines the vertical stability of the atmospherei.e., the tendency of an air particle to return to its original position or to accelerate away from its original position after being given a slight vertical displacement. For this reason, the lapse rate is of prime importance to meteorologists in forecasting certain types of cloud formations, the incidence of thunderstorms, and the intensity of atmospheric turbulence.