environmental lapse rate

Rate at which temperature changes with altitude. Usually, because the atmosphere is heated from below, temperatures decrease as altitude increases at an average rate of −6.4°/11.5°F per 1,000 m/3,280 ft. The rate varies from place to place and according to the time of day.

Under certain conditions, the lapse rate can be reversed, so that temperatures increase with altitude. This can be caused by large-scale atmospheric movements, for example where a warm stable air mass overlies a cool, less stable air mass. This is common in trade wind areas of the tropics. Smaller scale temperature inversions develop at night where surface temperatures fall rapidly, cooling the air near the surface. It can also be caused by cold air flowing down a slope into valleys.