Page 299 - Fundamentals of Air Pollution

II. Stability and Instability 255

Fig. 17-7. Temperature of a parcel of air forced to rise 200 m compared to an inversion
environmental lapse rate. Since the parcel is cooler than the environment, it will sink back
to its original level.

at a given time may be estimated by use of the morning radiosonde ascent
plotted on a thermodynamic chart. The surface temperature at the given
time is plotted on the diagram. If a dry adiabat is drawn through this
temperature, the height aboveground at the point where this dry adiabat
intersects the morning sounding is the mixing height for that time. The
mixing height for the time of maximum temperature is the maximum mixing
height. Use of this sounding procedure provides an approximation because
it assumes that there has been no significant advection since the time of
the sounding.

D. Potential Temperature

A useful concept in determining stability in the atmosphere is potential
temperature. This is a means of identifying the dry adiabat to which a
particular atmospheric combination of temperature and pressure is related.
The potential temperature 0 is found from

where T is temperature and p is pressure (in millibars, mb). This value is
the same as the temperature that a parcel of dry air would have if brought
dry adiabatically to a pressure of 1000 mb.
If the potential temperature decreases with height, the atmosphere is
unstable. If the potential temperature increases with height, the atmosphere
is stable. The average lapse rate of the atmosphere is about 6.5°C/km; that
is, the potential temperature increases with height and the average state
of the atmosphere is stable.