Geoscience Reference
In-Depth Information
4
CHAPTER FOUR
Atmospheric moisture
budget
LEARNING OBJECTIVES
When you have read this chapter you will:
n
be familiar with the major atmospheric components of the hydrological cycle
n
know the main controls of evaporation and condensation
n
be aware of the spatial and temporal characteristics of moisture in the atmosphere, evaporation
and precipitation
n
know the different forms of precipitation and typical statistical characteristics
n
know the major geographical and altitudinal patterns of precipitation and their basic causes
n
understand the nature and characteristics of droughts.
This chapter considers the role of water in its
various phases (solid, liquid and vapor) in the
climate system and the transfers (or cycling) of
water between the major reservoirs - the oceans,
the land surface and the atmosphere. We discuss
measures of humidity, large-scale moisture
transport, moisture balance, evaporation and
condensation.
atmosphere. The oceans, with a mean depth of
3.8km and covering 71 percent of the earth's
surface, hold 97 percent of
all
the earth's water
(23.4
10
6
km
3
). Approximately 70 percent of the
total
fresh
water is locked up in ice sheets and
glaciers, while almost all of the remainder is
ground water. It is an astonishing fact that
rivers and lakes hold only 0.3 percent of all fresh
water and the atmosphere a mere 0.04 percent
(
Figure 4.1
). The average residence time of water
within these reservoirs varies from hundreds or
thousands of years for the oceans and polar ice to
only about 10 days for the atmosphere. Water
cycling involves evaporation, the transport of
water vapor in the atmosphere, condensation,
precipitation and terrestrial runoff. The equations
of the water budget for the atmosphere and for the
surface are respectively:
×
A THE GLOBAL
HYDROLOGICAL CYCLE
The global hydrosphere consists of a series of
reservoirs interconnected by water cycling in
various phases. These reservoirs are the oceans;
ice sheets and glaciers; terrestrial water (rivers,
soil moisture, lakes and ground water); the
biosphere (water in plants and animals); and the
