Fig. 1.

The dry adiabatic lapse rate (blue line) is 5.4F per 1000 feet. Any lapse rate in which the temperature decreases at a rate greater than 5.4F per 1000 feet is absolutely unstable. The moist or saturated adiabatic lapse rate (red line) is approximately 2.0F per 1000 feet. Any lapse rate in which the temperature decreases at a rate less than 2.0F per 1000 feet, or if the temperature increases with altitude, is absolutely stable. Any decrease of temperature with increase in altitude that falls between the moist and the dry adiabatic lapse rates indicates that the parcel of air is conditionally either stable or unstable depending on its moisture content. If it is dry air then it is conditionally stable. If it is moist or saturated air then it is conditionally unstable.

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Copyright 1996 Harold F. Maybeck. All Rights Reserved.