The environmental lapse rate of temperature in an atmosphere in which the density is constant with height (homogeneous atmosphere), equal to g/R, where g is the acceleration of gravity and R the gas constant. For dry air the autoconvective lapse rate is approximately +3. 4 × 10−4°C per cm. Although the term is poorly named (see autoconvection), it is useful in optics, since it defines the transition between upward and downward refraction of light in the atmosphere.