Environmental Lapse Rate
•The environmental (or ambient) lapse rate is referred to the vertical change in temperature through still air.
•The environmental lapse rate is not fixed. It changes from day to day and from place to place.
D:\yu\maker\talk\uci\class\ESS11\image\env.lapse.rate.gif
environmental lapse rate =0.5°C/100m
(from Understanding Weather & Climate)