The rate of change of temperature with altitude (). The rate of temperature decrease with increase in altitude which is expected to occur in an unperturbed dry air mass is . This is called the dry adiabatic lapse rate. The lapse rate is taken as positive when temperature decreases with increasing height. For air saturated with H₂O, the lapse rate is less because of the release of the latent heat of water as it condenses. The average tropospheric lapse rate is about . The lapse rate has a negative value within an inversion layer.