Stability atmospheric

The lapse rate in the lower portion of the atmosphere has a great influence on the vertical motion of air. If the lapse rate is adiabatic, a parcel of air displaced vertically is always at equilibrium with its surroundings. Such a condition, in which vertical displacements are not affected by buoyancy forces, is called neutral stability. However, because of surface heating and local weather influences, the atmosphere seldom has an adiabatic temperature profile. The atmosphere is either  

Unstable—buoyancy forces enhance vertical motion. 

Stable—buoyancy forces oppose vertical motion. 

Let us suppose a warm parcel begins to rise in an atmosphere in which temperature decreases more rapidly with z than the adiabatic rate (its lapse rate exceeds the adiabatic lapse rate). The air parcel cools adiabatically, but the temperature difference between the rising parcel and the surroundings increases with z. If the density of the parcel is p and that of the air p the acceleration experienced by the parcel is 

Thus the acceleration increases with z and the parcel continues to rise as long d T T. We can express the acceleration in terms of the two lapse rates F and A as follows, if 

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. 

FIGURE 16.3 Regimes of absolute stability, absolute instability, and conditional stability in the atmosphere. 

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. 

FIGURE 16.4 Temperature profile and pollutant mixing for (a) nighttime radiation inversion and (b) subsidence inversion. 

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This solution describes a plume with a Gaussian distribution of poUutant concentrations, such as that in Figure 5, where (y (x) and (y (x) are the standard deviations of the mean concentration in thejy and directions. The standard deviations are the directional diffusion parameters, and are assumed to be related simply to the turbulent diffusivities, and K. In practice, Ct (A) and (y (x) are functions of x, U, and atmospheric stability (2,31—33). 

Lapse Rate and Atmospheric Stability Apart from mechanical interference with the steady flow of air caused by buildings and other obstacles, the most important fac tor that influences the degree of turbulence and hence the speed of diffusion in the lower air is the varia-... 

FIG. 26-31 Estimated maximum downwind distance to lower flammable limit L, percent by volume at ground level in centerline of vapor cloud, vs. continuous dense vapor release rate at ground level. E atmospheric stability. Level terrain. Momentary concentrations for L. Moles are gram moles u is wind speed. (From Bodmtha, 1980, p. 105, by permission.)... 

Parameters Affeeting Gas Dispersion A wide variety of parameters affect the dispersion of gases. These include (1) wind speed, (2) atmospheric stability, (3) local terrain characteristics, (4) height of the release above the ground, (5) release geometry, i.e. from a point, line, or area source, ( momentum of the material released, and (7) buoyancy of the material released. 

TABLE 26-28 Atmospheric Stability Classes for Use with the Pasquill-Gifford Dispersion Model... 

The Britter and McQiiaid model was developed by performing a dimensional analysis and correlating existing data on dense cloud dispersion. The model is best suited for instantaneous or continuous ground-level area or volume source releases of dense gases. Atmospheric stability was found to have little effect on the results and is not a part of the model. Most of the data came from dispersion tests in remote, rural areas, on mostly flat terrain. Thus, the results would not be apphcable to urban areas or highly mountainous areas. 

The atmospheric stability class The lapse rate SfYJdz)... 

Plume rise observations based on single-stack operation were regressed into the above expression and empirically fitted to the following expression, which incorporates atmospheric stability classes into the coefficients ... 

Time, Hours Wind Speed, m/s Atmospheric Stability Class... 

Critical GLC s can usually be calculated based on a unstable atmosphere, thus enabling the designer to determine a worst case scenario. For any given day, typical atmospheric stabihty data can usually be obtained from a local weather bureau, or may be estimated from the so-called Pasquill chart for the appropriate Atmospheric Stability Class (refer to Table 1). 

The determination of the critical GLC is a trial and error computation of GLC s due to various wind speeds, atmospheric stabilities and downwind distances. The maximum value obtained from these procedures is the critical GLC. Because of the number of computations involved, calculations should be performed on the computer. Software simulation is also necessary to calculate GLC s due to multiple stack cases. Wind direction is an additional variable that must be taken into account with multiple stact cases. 

The weather data are based on thousands of observations of wind speed, wind direction and atmospheric stability taken over the desired averaging interval at local weather bureau stations. 

From Figures 2 and 3, the Pasquill-Gifford dispersion coefficients are obtained for a downwind distance of 2000 meters and for atmospheric stability Class B. 

Atmospheric stability The state of the atmosphere in which vertical air movement is restricted. 

Note also that the relative sizes cf the altered zones are not to scale (e.g., choosing a higher value for the level of concern does not always result in a smaller zone than the use of greater wind speed and less atmospheric stability). 

Atmospheric stability and mechanical turbulence (important near to the ground) are used to derive the vertical and horizontal dispersion coefficients. Table 45.2 shows Pasquill s stability categories used to derive the coefficients by reference to standard graphs. 

In the lower levels of the atmosphere, the ELR changes with the time of day. Figure 25.33 shows a typical daily variation of atmospheric stability. Starting before sunrise, the minimum temperature is at the surface. This is caused... 

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