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. For air parcels of differing water content this value is different. In the troposphere, these two values are generally inversely proportional: increasing altitude means a lower temperature until the tropopause is reached. When altitude increases and temperature decreases, the lapse rate is considered positive, and for unsaturated tropospheric air this is approximately 9. 8 degrees C/km; however, in an inversion layer the atmospheric lapse rate is negative, that is, the atmosphere's temperature warms with increasing altitude.