cooling of air and reaches its maximum in the upper levels of the troposphere. Temperature decrease with height is described by the lapse rate. On the average, temperature decreases —0.65°C/100 m or —6.5°C/km. This decrease is the normal lapse rate. If warm dry air is lifted in a dry environment, it undergoes adiabatic expansion and cooling. This adiabatic cooling results in a lapse rate of — 1°C/100 m or — 10°C/km, the dry adiabatic lapse rate.

Individual vertical temperature measurements vary from either the normal or dry adiabatic lapse rate. This change of temperature with height for measurement is the environmental lapse rate. Values for the environmental lapse rates characterize the stability of the atmosphere and profoundly affect vertical air motion and the dispersion of pollutants (Godish 1991).

If the environmental lapse rate is greater than the dry adiabatic lapse rate, dispersion characteristics are good to excellent. The greater the difference, the more unstable the atmosphere and the more enhanced the dispersion. If the environmental lapse rate is less than the dry adiabatic lapse rate, the atmosphere becomes stable, and dispersion becomes more limited. The greater the difference from the adiabatic lapse rate, the more stable the atmosphere and the poorer the dispersion potential (Godish 1991).

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