Actual lapse rate

If potential temperature varies with height, then the actual lapse rate (F = — W H differ from the dry adiabatic lapse rate. This difference... 

If A is the actual lapse rate in the atmosphere, then at any height z... 

When the actual temperature-decline-with-altitude is greater than 9.8°C/1000 m, the atmosphere is unstable, the Cj s become larger, and the concentrations of poUutants lower. As the lapse rate becomes smaUer, the dispersive capacity of the atmosphere declines and reaches a minimum when the lapse rate becomes positive. At that point, a temperature inversion exists. Temperature inversions form every evening in most places. However, these inversions are usuaUy destroyed the next morning as the sun heats the earth s surface. Most episodes of high poUutant concentrations are associated with multiday inversions. 

The potential temperatiue 0 is that to which dry air originally in the state (T, p) would come if brought adiabatically to po- Adiabatic temperature profiles expressed in terms of 0 are vertical on a plot of z vs 0, facilitating comparisons of actual temperature profiles to the adiabatic lapse rate. 

Figure 4. Projection of sea level temperature changes (MAT and MART) from coastal to interior areas for two transects in the western United States. Transect A 1 = coastal California, 2 = southeastern California, 3 = northern Arizona Transect B 4 = coastal Washington, 5 = Washington, 6 = Idaho, 7 = central Montana. Points 1 and 4 represent actual values all other values are calculated from local lapse rates to 0 m. From Meyer (1992). [Reprinted from Palaeogeography, Palaeoclimatology, Palaeoecology, v. 99, Meyer, H.W., Lapse rates and other variables applied to estimating paleoaltitudes from fossil floras, p. 71-99, 1992, Elsevier Science Publishers B.V., with permission from Elsevier.]...

Even if actual evaporation rates for the sites in this study are difficult to predict because of the dependence on the specific microclimatic conditions, we can infer general trends for the altitudinal transects in New Zealand and California. Both are located in temperate areas and have relatively dry temperature lapse rates (6 °C), and in California, cloudiness increases with altitude. Comparing these conditions to the modeled environments discussed above would suggest that evaporation is likely to decrease with altitude, or at least not increase significantly. The larger leaf size of the oak leaves may increase their evaporation rates relative to the smaller mountain beech leaves, but this remains speculative as no irradiation data available were available for either site. 

Conversely, if the actual measured lapse rate is greater than 9.8°C/1000 m, a parcel of air displaced upward from its initial height becomes warmer than its surroundings and therefore tends to rise (Fig. 4-6b). If pushed downward, the parcel becomes colder than its surroundings and therefore tends to keep sinking. In this case, buoyant forces amplify any initial upward or downward movement of the air parcel, thus creating an unstable atmosphere. 

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 temperature 9 defined by (14.12) is called the potential temperature. We introduce the potential temperature because an actual atmosphere is seldom adiabatic and we want to relate the actual temperature profile to the adiabatic lapse rate. Adiabatic temperature profiles based on potential temperature are vertical on a plot of c versus 0, thereby facilitating such comparisons. 

One might ask Why does not the atmosphere always have an adiabatic lapse rate as its actual profile The reason it does not is that other processes such as winds and solar heating of the Earth s surface lead to dynamic temperature behavior in the lowest layers of the atmosphere that is seldom adiabatic. These other processes exert a much stronger influence on the prevailing temperature profile than does the adiabatic rising and failing of air parcels. 

Thus a cloudy atmosphere is inherently less stable than a dry atmosphere, and a stable situation with reference to the dry adiabatic lapse rate may actually be unstable for upward displacements of a saturated air parcel. 

The actual temperature profile of the ambient air shows the environmental lapse rate. Sometimes called the prevailing or atmospheric lapse rate, it is the result of complex interactions of meteorological factors and is usually considered to be a decrease in temperature with height. It is particularly important to vertical motion because surrounding air temperature determines the extent to which a parcel of air rises or falls. 

---