Geoscience Reference
In-Depth Information
Table 1.2
Estimated continental water balance (Data from Korzun, 1978).
Runoff as
percentage of
precipitation
Precipitation
(mm year
−1
)
Evaporation
(mm year
−1
)
Runoff
(mm year
−1
)
Africa
740
587
153
21
Asia
740
416
324
44
Australia
791
511
280
35
Europe
790
507
283
36
North America
756
418
339
45
South America
1595
910
685
43
in time and space. It is helpful to recognize the nature of these components from
the outset and to appreciate in general terms how they influence global hydrocli-
matology. For this reason we next consider salient features of the atmosphere,
hydrosphere, cryosphere, and lithosphere, biosphere, and anthroposphere.
Atmosphere
The air surrounding the Earth is a mixture of gases, mainly (~80%) nitrogen and
(~20%) oxygen, but also other minority gases such as carbon dioxide, ozone, and
water vapor which have an importance to hydroclimatology not adequately
reflected by their low concentration. Compared to the diameter of the Earth
(~20,000 km), the depth of the atmosphere is small. The density of air changes
with height but about 90% of the mass of the atmosphere is within 30 km and
99.9% within 80 km of the ground.
The atmosphere is (almost) in a state of hydrostatic equilibrium in the vertical,
with dense air at the surface and less dense air above; there is an associated change
in pressure. The temperature of the air changes with height in a very distinctive
way and this can be used to classify different layers or 'spheres. Figure 1.2 shows
the vertical profile of air temperature in the US Standard Atmosphere (US Standard
Atmosphere, 1976) as a function of height and atmospheric pressure. Starting
from the surface, the main layers are the troposphere, stratosphere, mesosphere,
and thermosphere, separated by points of inflection in the vertical temperature
profile that are called 'pauses. Near the ground, air temperature falls quickly with
height for reasons which are discussed in more detail later. Higher in the
atmosphere the air is warmed by the release of latent heat when water is condensed
in clouds and, in the upper stratosphere, it is also warmed by the absorption of a
portion of incoming solar radiation. There is then further cooling through the
mesosphere, but some further warming at the very top of the atmosphere where
most of the Sun's gamma rays are absorbed.
