Environmental Engineering Reference
In-Depth Information
because the air does not contain any moisture, but because no condensation has taken
place.
We have, therefore, a framework for determining how far a bubble will rise.
So long
as no condensation occurs
, the warmer, rising air of the bubble will cool at the dry
adiabatic lapse rate. The surrounding air changes temperature at the environmental lapse
rate. The bubble rises until its temperature (and therefore its density) is equal to that of
the surrounding air. This is shown diagramatically in Figure 4.1.
As the dry adiabatic lapse rate is a constant, the two variables in this relationship are
the environmental lapse rate and the initial temperature of the air bubble. The bubble will
rise only if it is warmed sufficiently to overcome the restraining effect of the
environmental lapse rate. If the bubble cannot rise it is said to be stable. If the
temperature is raised far enough, however, or if the environmental lapse rate is great
enough, the air bubble can rise a considerable distance. It is in these circumstances that
part of the troposphere is said to be unstable.
CONDENSATION
'So long as no condensation occurs …' That was the proviso we established when
considering the dry adiabatic lapse rate. But all air, even in the driest desert, does contain
some moisture.
The ability of the air to hold moisture is dependent upon its temperature. Water
molecules are continuously changing from one state to another. If more molecules are
leaving a liquid surface than arriving, net evaporation is experienced; if more arrive than
leave there is net condensation. As the temperature of the air increases so its moisture-
holding capacity also rises; or, to put it another way, more moisture must be added to
reach saturation at a higher temperature. If air is cooled, the evaporation rate will
decrease more rapidly than does the condensation rate. At a certain temperature,
evaporation becomes less than condensation and the water droplet grows. This
temperature is known as the dew point and varies with moisture content.
The amount of moisture which air can hold may be assessed in a number of ways.
Relative humidity
is the most frequently used term. It is the ratio of the amount
