Atmosphere adiabatic lapse rate

ADIABATIC LAPSE RATE ATMOSPHERIC LAPSE RATE... 

Dry air rising in the atmosphere has to expand as the pressure in the atmosphere decreases. This pV work decreases the temperature in a regular way, known as the adiabatic lapse rate, Td, which for the Earth is of order 9.8 Kkm-1. As the temperature decreases, condensable vapours begin to form and the work required for the expansion is used up in the latent heat of condensation of the vapour. In this case, the lapse rate for a condensable vapour, the saturated adiabatic lapse rate, is different. At a specific altitude the environmental lapse rate for a given parcel of air with a given humidity reaches a temperature that is the same as the saturated adiabatic lapse rate, when water condenses and clouds form Clouds in turn affect the albedo and the effective temperature of the planet. Convection of hot, wet (containing condensable vapour) air produces weather and precipitation. This initiates the water cycle in the atmosphere. Similar calculations may be performed for all gases, and cloud layers may be predicted in all atmospheres. 

The adiabatic lapse rate for a dry atmosphere, which may contain water vapor but which has no liquid moisture present in the form of fog, droplets, or... 

It is important not to confuse this dry adiabatic lapse rate with the rate of change in temperature with height in a Standard Atmosphere. The latter represents average conditions in Earth s atmosphere, where heating, mixing, and wet adiabatic processes also are occurring. 

FIGURE 4-10 Emission of pollutants from a smokestack, a typical continuous source, under a variety of meteorological conditions. The dry adiabatic lapse rate is represented as a dashed line and the actual measured lapse rate as a solid line in the left panels. Vertical mixing is strongest when the adiabatic lapse rate is less than the actual measured lapse rate and the atmosphere is unstable (top). Weak lapse is a term used to express the existence of a stable atmosphere, which results in less vigorous vertical mixing. An inversion, in the third panel from the top and in part of the last three panels, results in a very stable atmospheric layer in which relatively little vertical mixing occurs (Boubel et al, 1994). 

The regions of the atmosphere are defined by the vertical temperature profile. At the bottom is the troposphere where temperature decreases with height from the surface (which is warmed by the sun). The rate of change of temperature (the lapse rate) depends on the amount of moisture in the air since the latent heats of condensation and evaporation affect the heat of a rising or descending air parcel. For dry air the dry adiabatic lapse rate is — 9.8 K km but a more typical value of the environmental lapse rate (for air containing some water vapor) is — 6.5 K km The troposphere extends up to about 10 km, though this varies with... 

An important factor in determining the extent of air pollution in urban areas is whether the atmosphere is stable (poor mixing, accumulation of pollutants) or unstable (good mixing and dispersion of pollutants). Whether the atmosphere is stable or unstable depends on how the temperature profile (the so-called lapse rate) in the atmosphere near ground level compares with the adiabatic lapse rate. The adiabatic lapse... 

Adiabatic Lapse Rate of an Air Parcel Containing Water Vapor The atmosphere contains also water vapor with heat capacity at constant pressure equal to 1952 J kg 1 K 1. Calculate the adiabatic lapse rate T for an air parcel as a function of its water vapor content. What is the maximum expected difference between r and T ... 

FIGURE 16.2 Temperature profiles for (a) an unstable atmosphere and (b) a stable atmosphere. The dry adiabatic lapse rate is also shown. 

Our previous discussion focused on a rising air parcel. The same conclusions are applicable to a sinking air parcel. If A > T, then the atmosphere enhances its motion, whereas if A < T, the atmosphere suppresses it. Finally, if the air parcel is saturated with water vapor, one would need to use the moist adiabatic lapse rate Ts instead of T in the discussion above. 

The lapse rate in the lower portion of the atmosphere has a great influence on the vertical motion of air. Buoyancy can resist or enhance vertical air motion of airmasses, thus affecting the mixing of pollutants. Comparing the environmental lapse rate A with the adiabatic lapse rate T, we can define three regimes of atmospheric stability (Figure 16.2) ... 

If the environmental lapse rate lies between the dry and moist values, then the stability of the atmosphere depends on whether the rising air is saturated. When an air parcel is not saturated, then the dry adiabatic lapse rate is the relevant reference state and the atmosphere is stable. For a saturated air parcel inside a cloud, the moist adiabatic rate is the applicable criterion for comparison and the atmosphere is unstable. Therefore a cloudy atmosphere is inherently less stable than the corresponding dry atmosphere with the same lapse rate. 

Let us assume that at some initial time t = 0 relative to the adiabatic atmosphere. Then, from (16.A. 14) we see that if Q = 0, the condition of T = 0 is preserved for t > 0 even though there may be motion of air. Also, the equation of motion (16.A.11) reduces to the usual form of the Navier-Stokes equation for the dynamics of an incompressible fluid under the influence of a motion-induced pressure fluctuation p with no contribution from buoyancy forces since T = 0. Therefore, for an atmosphere with no sources of heat and initially having an adiabatic lapse rate, the temperature profile is unaltered if the atmosphere is set in motion. As a result, the adiabatic condition can be envisioned as one in which a large number of parcels are rising and falling, a sort of convective equilibrium. Thus, we have been able to derive the relation for the adiabatic lapse rate here from the full equation of continuity, motion, and energy, in contrast with the derivation presented in Section 16.1.1, which is based on thermodynamic arguments. 

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