The rate of decrease of temperature with increasing altitude in the atmosphere. If heat is neither gained nor lost from the air parcel under consideration, then the lapse rate is said to be adiabatic and the energy to expand the volume of the parcel or rising air comes from the kinetic energy of the gas molecules in that parcel. The expansion of the parcel causes these molecules net kinetic velocity to decrease and this is equivalent to cooling the air. In dry air the dry adiabatic lapse rate is about 9.8 C/km (the sign is traditionally positive although the temperature is decreasing with altitude). In moist air the release of latent heat by condensation of the contained moisture makes the lapse rate lower, ~ 4 C/km. Surface temperature inversions are created when the lapse rate goes negative, that is, when the temperature profile of low lying tropospheric air increases with altitude. Air at the earth's surface is trapped there because the air above it is less warmer and less dense. This can be disastrous for the air quality in urban areas when anthropogenic pollutants are not mixed away from the surface and, instead, build up. [Graedel, T.D. and Paul Crutzen. Atmospheric Change: an Earth system perspective; 1993; Freeman Press.] [Monthly Weather Review; 135; 985-1005; 2007.] [Journal of Atmospheric Sciences; 64; 314-337; 2007.]