A lapse rate is the rate at which an atmospheric variable, usually air temperature, falls as altitude increases.
Rising dry air cools at about 9.8 °C per kilometer (5.4 °F per 1,000 feet), the dry adiabatic lapse rate, while saturated air cools more slowly, typically around 5 °C per kilometer. The International Standard Atmosphere uses an average environmental lapse rate of 6.5 °C per kilometer up to 11 km, and meteorologists compare these rates to judge whether air will rise and form clouds or storms.
Source: Lapse rate, Wikipedia