In the stratosphere, the temperature of the ambient air usually decreases with an increase in altitude. This rate of temperature change is called the elapse rate. Environmental engineers can determine this rate for a place and time by sending up a balloon equipped with a thermometer. The balloon moves through the air, not with it, and measures the temperature gradient of ambient air, called the ambient lapse rate, the environmental lapse rate, or the prevailing lapse rate.

Using the ideal-gas law and the law of conservation of energy, environmental engineers have established a mathematical ratio for expressing temperature change against altitude under adiabatic conditions (Petterssen 1968). This rate of decrease is termed the adiabatic elapse rate, which is independent of the prevailing atmospheric temperature.

Dry air, expanding adiabatically, cools at 9.8°C per km (or 5.4°F per 1000 ft), which is the dry adiabatic lapse rate (Smith 1973). In a wet as in a dry adiabatic process, a parcel of air rises and cools adiabatically, but a second factor affects its temperature. Latent heat is released as water vapor condenses within the saturated parcel of rising air. Temperature changes in the air are then due to the liberation of latent heat as well as the expansion of air. The wet adiabatic lapse rate (6°C/km) is thus less than the dry adiabatic lapse rate. Since a rising parcel of effluent gases is seldom completely saturated or completely dry, the adi-abatic lapse rate generally falls somewhere between these two extremes.

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