Adiabatic Lapse Rate
The adiabatic lapse rate refers to the rate at which the temperature of a parcel of air changes as it rises or descends in the atmosphere without exchanging heat with its surroundings. The dry adiabatic lapse rate is approximately 9.8°C per kilometer, while the moist adiabatic lapse rate varies depending on the moisture content of the air. Understanding these rates is crucial in meteorology and atmospheric science.
4 Key excerpts on "Adiabatic Lapse Rate"
eBook - ePub- Roger G. Barry, Richard J Chorley(Authors)
- 2009(Publication Date)
- Routledge(Publisher)
...When an air parcel moves vertically, the changes that take place are generally adiabatic, because air is fundamentally a poor thermal conductor, and the air parcel tends to retain its own thermal identity, which distinguishes it from the surrounding air. However, in some circumstances, mixing of air with its surroundings must be taken into account. Consider the changes that occur when an air parcel rises: the decrease of pressure (and density) causes its volume to increase and temperature to decrease (see Chapter 2 B). The rate at which temperature decreases in a rising, expanding air parcel is called the Adiabatic Lapse Rate. If the upward movement of air does not produce condensation, then the energy expended by expansion will cause the temperature of the mass to fall at the constant dry Adiabatic Lapse Rate or DALR (9.8°C/km). However, prolonged cooling of air invariably produces condensation, and when this happens latent heat is liberated, counteracting the dry adiabatic temperature decrease to a certain extent. Therefore, rising and saturated (or precipitating) air cools at a slower rate (the saturated Adiabatic Lapse Rate or SALR) than air that is unsaturated. Another difference between the dry and saturated adiabatic rates is that whereas the DALR is constant the SALR varies with temperature. This is because air at higher temperatures is able to hold more moisture and therefore on condensation releases a greater quantity of latent heat. At high temperatures, the saturated Adiabatic Lapse Rate may be as low as 4°C/km, but this rate increases with decreasing temperatures, approaching 9°C/km at –40°C. The DALR is reversible (i.e., subsiding air warms at 9.8°C/km); whereas in any descending cloud air saturation cannot persist because droplets evaporate. Three different lapse rates must be distinguished: two dynamic and one static...
eBook - ePubEnvironmental Engineering
Fundamentals and Applications
- Subhash Verma, Varinder S. Kanwar, Siby John(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
...The dry Adiabatic Lapse Rate (DALR) is given by: (d T d Z) a d = − 9.86 ° C k m = − n − 1 n × g R The dry Adiabatic Lapse Rate is 1 0 C/100 m. Dry Adiabatic Lapse Rate is important in defining atmospheric stability. A comparison of the adiabatic rate to the environmental lapse rate indicates the stability of the atmosphere. Atmospheric stability is a measure of the ability of the atmosphere to disperse pollutants emitted into it. 32.1.3 Stability Conditions Several possible environmental lapse rates are compared with the Adiabatic Lapse Rate in Figure 32.2. Figure 32.2 Atmospheric Stability Conditions When the prevailing lapse rate, ambient lapse rate or environmental lapse rate is greater than the dry Adiabatic Lapse Rate, the atmosphere is said to be superadiabatic or unstable. This is because any perturbation in the vertical direction tends to be enhanced. Therefore, it is a favourable atmospheric condition with respect to the dispersion of pollutants. When the environmental lapse rate is approximately the same as the dry Adiabatic Lapse Rate, the stability of the atmosphere is said to be neutral. Any parcel of air carried rapidly upwards or downwards will have the same temperature as the environment at the new height. Hence there is no tendency for any further vertical movement. When the environmental lapse rate is less than the dry Adiabatic Lapse Rate, a rising air parcel becomes cooler and denser than its surroundings and tends to fall back to its original position. Such an atmospheric condition is called stable, and the lapse rate, which is subadiabatic, is negative. In such conditions, a dense cold stratum of air at ground level gets covered by lighter, warmer air at a higher level. This phenomenon is called inversion. During inversion, vertical air movement is stopped and the pollution is concentrated beneath the inversion layer, i.e., the denser air at ground level...
eBook - ePub- Mike Wickson(Author)
- 2014(Publication Date)
- Crowood(Publisher)
...At heights where temperatures are low, the clouds which form are very thin because there is little water vapour in the air although saturated, to condense out and form the cloud. At these heights close to the tropopause, the heating effect due to condensation is very small and in fact SALR and DALR (Dry Adiabatic Lapse Rate) are both 3°C per 1,000 ft. The same can apply at the surface at high latitudes where again temperatures can be very low. Descending Air The case of air being lifted and the subsequent lapse rates is important because it has a direct bearing on cloud formation. Air can be made to descend as well as to rise. This occurs in clouds and also with anticyclones. In this latter case the air diverges from the centre of the anticyclone at the surface causing subsidence of the air above. The result is adiabatic warming of the descending air as it is forced to a lower level where the pressure is greater. As previously mentioned this compression of the air can cause a temperature inversion. Stability If a volume of air is blown against a building and lifted, when that air has moved away from the buildings and is in the lee, one of two things will happen. Either the air will tend to sink back to the ground, or alternatively, it will continue to rise. These two conditions are representative of atmospheric conditions which are termed stable or unstable respectively. The atmosphere is said to be stable, if when air is lifted and the lifting force is removed the rising air tends to return to its original level. This situation must appertain if the lifted volume of air as it rises, becomes colder than the atmosphere at the same level. The volume would then be denser and heavier than the surrounding air, which we can term the environmental air, and will therefore try to sink. The lapse rate in the surrounding air within which a volume of air is being lifted is termed the environmental lapse rate...
eBook - ePub- Gino C. Segrè, John D. Stack(Authors)
- 2022(Publication Date)
- University of Chicago Press(Publisher)
...At first sight it is worrisome that Eqs. (3.45) and (3.50) predict zeroes in pressure and temperature at z = 7 D 0 / 2. However, this altitude is well above the troposphere, and into the next layer of the atmosphere, the stratosphere, where additional factors come into play. Lapse Rate Remaining in the troposphere, Eq. (3.50) predicts a definite value for the “lapse rate,” Γ, which is Using Eqs. (3.31) and (3.50), we have Using the definitions of C P, C V, and γ, Γ becomes the very simple expression, This expression for Γ, which is known as the “dry air lapse rate,” is the ratio of the force of gravity on a mole of air, divided by the specific heat/mole at constant pressure. Its numerical value is - 9.8K / km, so an increase in altitude of 1 km will be accompanied by a decrease in temperature of 9.8K. While this is accurate for dry air, the actual air in the atmosphere is rarely completely dry. The presence of water vapor in the atmosphere has a large effect on the lapse rate, generally reducing it from the dry air lapse rate to around 6K / km. The treatment of an adiabatic atmosphere containing water vapor is a more complex subject which is treated in Sec. (3.7). The lapse rate is closely related to D 0. From Eqs. (3.31) and (3.53), we have From the adiabatic formula for pressure, Eq. (3.45), we have P/P 0 = 0.58 at z = D 0 / 2, and P/P 0 = 0.31 at z = D 0. At smaller values of z, the adiabatic formula approaches the isothermal formula for pressure. Again, the adiabatic formula for pressure, as expressed in Eq. (3.45), is a dry air formula. The atmospheres over a desert or a snowfield are examples of places where it would hold. 3.5 Chemical Potential. Thermodynamic Potentials Chemical Potential The cases discussed so far are those of a gas with a fixed number of particles. Thermodynamics applies to the more general case when the number of particles is varied. The corresponding intensive variable is called the “chemical potential” and is usually denoted as μ...
