The fall in temperature of stationary air with height, averaging 6 °C per 1 000 m. This fall of temperature with height in stable air is due to a fall in the density of the air, varying greatly with the time of day, and with the nature of the air mass concerned. See Schultz et al. (2000) Monthly Weather Rev. 128, 4143–8, and Houston and Niyogi (2007) Monthly Weather Rev. 135, 3013–32.