Environmental Engineering Reference
In-Depth Information
Chapter
4
Moisture in the atmosphere
On a hot day the picture of clouds building up often signifies that a storm is imminent.
We do not always appreciate what is happening, but these growing clouds represent one
of the most vital processes in the atmosphere - the condensation of water as it is raised to
higher levels and cooled within strong updraughts of air. The water, of course, was
derived from the surface - evaporated from the oceans, from the soil, or transpired by the
vegetation. But within the atmosphere a variety of events combine to convert the water
vapour, which is produced by evaporation, to water droplets. The air must rise and cool
for condensation to occur. In this chapter we shall be looking at the nature and
consequences of these processes. Precipitation at the ground surface and the evaporation
process will be covered in Chapter 5.
THE EFFECTS OF HEATING AND COOLING IN THE
ATMOSPHERE
GENERAL EFFECTS
The atmosphere is a highly complex system, and the effects of changes in any single
property tend to be transmitted to many other properties. Thus heating and cooling of the
air cause adjustments in relative humidity and buoyancy; they may cause condensation
and evaporation, cloud formation and the development of storms.
What happens, then, when air is heated? To simplify the problem we will consider a
parcel of air in contact with the ground. As the ground is heated, the air in contact with it
will warm also; its temperature rises and it expands. Gases expand on heating more than
liquids or solids, so this effect is quite marked. Moreover, as the air expands its density
falls; in simple terms the same mass of air now occupies a larger volume. As its density
falls so it becomes lighter than the surrounding air and it tends to rise. Reverse the
process, cool the parcel of air, and the opposite occurs. It contracts, its density increases
and it sinks.
One effect of the heating and cooling of the surface atmosphere is therefore to cause
vertical movements of air. But there are other effects. As the air becomes cooler its
ability to hold moisture in the form of water vapour is reduced. If it cools to the point
where it can no longer hold the water vapour in vapour form, condensation occurs and
water droplets appear. If the air is heated, these droplets tend to evaporate and become
water vapour once more. Thus heating and cooling are intimately linked with the
processes of evaporation, condensation and precipitation formation. Let us consider these
main effects in turn.
