Environmental Engineering Reference
In-Depth Information
KEY POINTS
1
The process of evaporation supplies moisture into the lower atmosphere. The prevailing
winds then circulate the moisture and mix it with drier air elsewhere. Only if we have
a dry surface in areas well away from the oceans and where dry subsiding air is
dominant will moisture levels be low.
2
Water vapour is only the first stage of the precipitation chain; the vapour must be
converted into liquid or solid forms. This is usually achieved by cooling, either
rapidly, as in convection, or slowly, as in cyclonic storms; mountains also cause uplift
but the rate will depend upon their shape, their height and the direction of the wind.
3
Even this is insufficient, as we can tell from the large number of clouds in the sky
which never give precipitation. To produce precipitation, the cloud droplets must
become large enough to reach the ground without evaporating. The cloud must possess
the right microphysical properties to enable the droplets to grow. It must have ice
crystals if the Bergeron-Findeisen process is to operate, or a wide spectrum of drop
sizes with plenty of moisture condensing for the collision - coalescence system to
work.
4
Even these suitable conditions may be insufficient if the cloud does not last long
enough for growth to take place. Clearly precipitation results from a delicate balance
of counteracting forces, some leading to droplet growth, others to droplet destruction.
Nevertheless, where conditions are basically favourable - where air can rise high
enough to produce large vertical developments of cloud - copious amounts of
precipitation can occur.
5
Conditions favouring rising air that give rise to precipitation most frequently occur
when the atmosphere is relatively unstable, when surface convergence is dominant or
when air is forced to rise over mountains. The origins of precipitation can be identified
as convection, convergence or orography, though they are interdependent.
