the condition of the atmosphere when the environmental lapse rate is less negative than the (dry-) adiabatic lapse rate, resulting in an increase in air temperature with altitude, i.e., δT/δz > –9.77°C/1,000 m (–5.4°F/1,000 ft), where T is the air temperature and z is height —note 1. the term inversion commonly denotes a condition where δT/δz > 0.0 —note 2. the layer in which this condition occurs is called the inversion layer, and the lowest altitude at which the departure is found is called the base of the inversion This definition last updated 10/23/2008