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DRY ADIABATIC LAPSE RATE. If a par-cel 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 1C per 100 meters of lift (10C per Km), or 5 1/2F per 1,000 feet of lift. If that parcel descends again to higher pressure, its temperature then INCREASES at the rate of 1C

Table 2-4-1.Lapse rates of temperature




Figure 2-4-3.Adiabatic lapse rates.

per 100 meters or 5 1/2F 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 1/2F 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 2 to 3F per 1,000 feet instead of 5 1/2F 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.

How the temperature of a parcel of air changes in response to these processes was illustrated in Unit 1 and because of its importance, it is illustrated again now. Assume that a saturated parcel of air having a temperature of 44F is at 5,000 feet and is forced over a 12,000-foot moun-tain. Condensation occurs from 5,000 to 12,000 feet so that the parcel cools at the moist adiabatic rate (3F per 1,000 ft) and reaches a temperature of approximately 23F at the top of the moun-tain. 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 moun-tain. When it reaches the 5,000-foot level, the parcel has descended 7,000 feet at a rate of 5 1/2F per 1,000 feet. This results in an increase of 38.5F. Adding the 38.5F increase to the original 12,000 feet temperature of 23F, the parcel has a new temperature of 61.5F.

AVERAGE ADIABATIC LAPSE RATE. The average lapse rate lies between the dry adiabatic and the moist adiabatic at about 3.3F per 1,000 feet.

SUPERADIABATIC LAPSE RATE. The superadiabatic lapse rate is a decrease in temperature of more than 5 1/2F per 1,000 feet and less than 15F per 1,000 feet.

AUTOCONVECTIVE LAPSE RATE. The autoconvective lapse rate is the decrease of more than 15F per 1,000 feet. This lapse rate is rare and is usually confined to shallow layers.

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