Temperature changes with an increase or decrease of altitude. This change is known as the "lapse rate" and it varies depending on the amount of moisture in the particular mass of air. The "dry adiabatic lapse rate" (for dry air masses) is a temperature decrease of about 3 degrees C per thousand feet of altitude, while the "wet adiabatic lapse rate" (for moist air masses) is a temperature decrease of about 1.66 degrees C per thousand feet of altitude.
For average conditions, a figure of 3.5 degrees F (2 degrees C) per 1000 feet is commonly used.
