Geoscience Reference
In-Depth Information
3
PRECIPITATION
Because the entire hydrologic cycle is basically driven by it, precipitation has to be con-
sidered the main component. Indeed, it is a truism that wherever there is no precipitation,
there is also not much of a hydrologic cycle. The detailed study of precipitation and of all
its aspects is properly the domain of meteorology. In hydrology, precipitation is primarily
of interest after it reaches the ground surface, and this is reflected in the organization of
this chapter. However, to gain a better understanding of the occurrence and distribution
of precipitation and its temporal and spatial scales, it is also useful to have a knowl-
edge of at least some elementary aspects of its generation mechanisms and of its major
types.
3.1
FORMATION OF PRECIPITATION
3.1.1
Mechanisms
Several processes take place jointly in the formation of precipitation. In brief, these are
the production of supersaturation of the air, condensation of water vapor into ice crystals
and droplets, the subsequent growth of these condensation products, and the supply of
moist air to where the first three processes occur. These processes involve a number of
different mechanisms, which are briefly reviewed in what follows.
Cooling of the air
As indicated in the previous chapter, the water-holding capacity of air decreases, as its
temperature decreases. Thus, air can become supersaturated by being cooled down. Such
cooling can occur by advection, for instance, as warmer air moves over a colder surface,
by radiative cooling or also as a result of the mixing of two different air masses; but these
are generally not very effective mechanisms and only capable at most of fog formation or
light drizzle. A more effective mechanism of cooling of the air consists of its being lifted
to higher elevations; in the generation of precipitation this is by far the main mechanism.
Air can be forced to rise by being heated from below, by moving over mountainous
terrain, or by frontal activity, that is by having to move over relatively heavier, that is
colder air. When the vertical air motions are relatively weak and gentle, for example
in the case of stable air in a warm front, the lifting may result in what is referred to
as
stratiform
precipitation. On the other hand, when the air is already unstable, and the
vertical motions are relatively strong, the resulting precipitation tends to be of the so-
called
convective
type. Under certain conditions the movement of air over mountainous
terrain can involve a combination of both stratiform and convective precipitation.
