Definition

The Lapse Rate is the rate at which temperature decreases with height in the Atmosphere. 

Lapse rate nomenclature is inversely related to the change itself: if the lapse rate is positive, the temperature decreases with height; conversely if negative, the temperature increases with height.

Description

When air is forced to rise up in the atmosphere, the pressure reduces with height. For a given volume of gas, the pressure divided by the temperature remains constant (Boyle's Law). Therefore, as the air pressure reduces, so does the temperature.

If no heat is exchanged with the surrounding air during this process, which is called “adiabatic cooling”, the rate at which the air cools, the Adiabatic Lapse Rate (ALR) is a constant.