DRY ADIABATIC LAPSE RATE

indicated by changes of lapse rate with height. (See table 2-1 and fig. 2-8.) DRY ADIABATIC LAPSE RATE.If a parcel of air is lifted, its pressure is DECREASED, since pressure decreases with height, and its temperature falls due to the expansion. If the air is dry and the process is adiabatic, the rate of temperature fall is 1°C per 100 meters of lift (10°C per Kin), or 5 l/2°F per 1,000 feet of lift. If that parcel descends again to higher pressure, its temperature then INCREASES at the rate of 1°C per 100 meters or 5 1/2°F per 1,000 feet. This is known as the dry adiabatic lapse rate. MOIST (SATURATION) ADIABATIC LAPSE RATE.When a mass of air is lifted, it cools at the dry adiabatic lapse rate of 5 1/2°F per 1,000 feet as long as it remains unsaturated (relative humidity below 100 percent). If the original moisture is being carried along with the mass as it ascends and it cools to its saturation temperature, the relative humidity reaches 100 percent. Condensation takes place with further cooling. For each gram of water condensed, about 597 calories of heat are liberated. This latent heat of condensation is absorbed by the air, and the adiabatic cooling rate is decreased to 20 to 3°F per 1,000 feet instead of 5 1/2°F per 1,000 feet. The process during the saturated expansion of the air is called the saturation adiabatic, the moist adiabatic, or the pseudoadiabatic process. The pseudoadiabatic process assumes that moisture falls out of the air as soon as it condenses. Assume that a saturated parcel of air having a temperature of 44°F is at 5,000 feet and is forced over a 12,000-foot mountain. Condensation occurs from 5,000 to 12,000 feet so that the parcel cools at the moist adiabatic rate (3°F per 1,000 ft) and reaches a temperature of approximately 23°F at the top of the mountain. Assuming that the condensation in the form of precipitation has fallen out of the air during the ascent, the parcel heats at the dry adiabatic rate as it descends to the other side of the mountain. When it reaches the 5,000-foot level, the parcel has descended 7,000 feet at a rate of 5 1/2°F per 1,000 feet. This results in an increase of 38.5°F. Adding the 38.5°F increase to the original 12,000 feet temperature of 23°F, the parcel has a new temperature of 61.5°F. AVERAGE ADIABATIC LAPSE RATE.The average lapse rate lies between the dry adiabatic and the moist adiabatic at about 3.3°F per 1,000 feet. SUPERADIABATIC LAPSE RATE.The superadiabatic lapse rate is a decrease in temperature of more than 5 1/2°F per 1,000 feet and less than 15°F per 1,000 feet. AUTOCONVECTIVE LAPSE RATE.The autoconvective lapse rate is the decrease of more than 15°F per 1,000 feet. This lapse rate is rare and is usually confined to shallow layers. 2-15 AG5t0201 Lapse rate Per 1,000 feet Per 100 meters Dry adiabatic Saturation (moist) adiabatic Average Superadiabatic Autoconvective 5 1/2 F 2-3 F 3.3 F 5 1/2-15 F More than 15 F 1 C .55 C .65 C 1-3.42 C More than 3.42 C Table 2-1.Lapse Rates of Temperature MOIST ADIABATIC AVERAGE LAPSE RATE "DRY" ADIABATIC SUPER ADIABATIC AUTO CONVECTIVE AG5f0208 Figure 2-8.Adiabatic lapse rates.

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