Geoscience Reference
In-Depth Information
Existing precipitating cloud
Feeder cloud
5000
500
4000
Drier
low-level
flow
400
3000
300
Moist
low-level
flow
2000
200
1000
100
0
0
Eskmeals
Boot
Hardknott
Pass
Sty Head
Borrowdale
Keswick Threlkeld
Penrith
Figure 4.10
Production of greater rainfall over hills as a result of forced ascent and extra seeding of the orographic cloud. The
cross-section is approximately from west to east across the English Lake District. The vertical bars indicate mean annual rainfall
at each location.
movement of water around the globe. Rainfall also takes
part in many of the processes that build our landscape;
plants and animals are also highly dependent upon
precipitation. Therefore in the next chapter we will
examine the question of types of precipitation and follow
its spatial variability on Earth's surface.
CONCLUSION
In this chapter we have followed the exchange and
movements of moisture between Earth's surface and the
atmosphere. Their effect upon the climate is obvious. But
precipitation is also important in other ways. It is a major
component of the hydrological cycle which portrays the
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 form. 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 range 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.
