Rate at which unsaturated air cools as it travels vertically, provided that all temperature change is adiabatic (without heat exchange), and no condensation occurs. In dry air it can be approximated as 9. 8 degrees Celsius per km of rise. It can be used as a basis of comparison for actual temperature profiles of air and can help predict smoke stack gas dispersion characteristics. Contrast with saturated adiabatic lapse rate which is something less than 9. 8 degrees/km of rise because of the release of heat at the air packet cools and water vapor condenses.