If air is forced to rise but has not cooled to its dew point temperature, the air will cool at a constant rate of about 10°C/1000m (5.5°F/1000 ft). This is known as the dry adiabatic rate (DAR). If the rising air cools to its dew point temperature, condensation commences; the air will continue to cool as it rises, but at a rate of about 6°C/1000m (3.2°F/1000 ft). This is known as the moist adiabatic rate (MAR). The MAR is less than the DAR because latent heat is released into the air during condensation. If the air stops its forced ascent, adiabatic cooling will also cease, and there would be no further condensation. If the air were forced to descend (e.g., on the leeward side of a mountain range), the air would heat adiabatically at the dry adiabatic lapse rate of 10°C/1000m. It is important not to confuse adiabatic cooling with the environmental lapse rate (ELR). The environmental lapse rate is simply the decrease in temperature with altitude through stratified air (air that is neither rising or descending) at a particular place and time. For adiabatic cooling or heating to occur, air must be forced to rise or sink, respectively. Summary: DAR = 10°C/1000m 6°C/1000m MAR = ELR varies depending on place and time. In the example below we will use 4.5°C/1000m

Applications and Investigations in Earth Science (9th Edition)

9th Edition

ISBN:9780134746241

Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa

Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa

Chapter1: The Study Of Minerals

Section: Chapter Questions

Problem 1LR

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Adiabatic Cooling and Heating. (See text pp. Section 6.3.) If a parcel of air is forced to ascend in altitude, it will expand with the lessened air pressure, and it will cool because of the decrease in the number of molecular collisions between air molecules. Alternatively, if the air parcel is forced to descend in altitude, it will be compressed with the increasing air pressure, and it will warm because of the increased number of molecular collisions. This is known as adiabatic cooling and heating. Upon lifting, the air may cool to its dew point. If so, the air has reached 100% relative humidity and therefore condensation occurs, clouds form, and with sufficient condensation some form of precipitation will result.