Environmental Engineering Reference
In-Depth Information
Figure 4.5
The effect of an inversion of temperature on cloud
development with moist air.
even if they cool at their maximum rate (the DALR), will be cooling more slowly than
the environment (c). If the environmental lapse rate is cooling more slowly than the
saturated adiabatic lapse rate we have absolute stability (a). If the environmental lapse
rate is between the DALR and the SALR we have conditional instability (b); in other
words, instability depends upon the air reaching saturation point.
Stability has a considerable effect upon the degree to which convective activity will
take place. If the air is unstable it will rise and may produce clouds, whereas if it is stable
convection will be reduced. Sometimes, especially under anticyclonic conditions, the
temperature will increase with height - a situation known as an inversion of temperature.
If the air beneath the inversion is fairly moist, a layer of cloud may develop here. Moist
air will have been brought to the inversion by convection and, as it cannot rise further, it
spreads out beneath the inversion to form a dense sheet of cloud (Figure 4.5).
Absolute instability in the atmosphere is infrequent except very close to the ground -
the convection it initiates helps to transfer heat upwards and so reduces the environmental
lapse rate. What is much more common is for the environmental lapse rate to lie between
the dry adiabatic lapse rate and the saturated adiabatic lapse rate. In this situation of
conditional instability the atmosphere is stable for air which has not reached saturation
point, but is unstable for saturated air. If the air can be forced to reach the condensation
level, either by ascent over hills or mountains, or by convergence associated with a
depression, it will become unstable and assist vertical motion. The former process is one
of the mechanisms which leads to higher rainfall over mountains.
CAUSES OF CONDENSATION
Clouds are one of the most interesting aspects of the sky. Their shape and form change
constantly to reflect the processes of formation and the environment in which they are
developing. To produce clouds, we need the air to reach saturation point. It is clear that
saturation can be reached either by cooling the air or by adding water to air (Figure 4.2).
It is by cooling of the air that the majority of clouds are formed. Orographic lifting,
convergent uplift near depressions or within air streams, and convection will all produce
vertical motion which may be sufficient to produce clouds. The second process of adding
