Geoscience Reference
In-Depth Information
Figure 3.30
Global distribution of the vertical transfer of sensible heat, in W m
-2
.
Source
: After Budyko
et al
. (1962).
exceeds 160 W m
-2
. It is less near the equator, where
wind speeds are somewhat lower and the air has a
vapour pressure close to the saturation value (see
Chapter 3A). It is clear from Figure 3.29 that the major
warm currents greatly increase the evaporation rate. On
land, the latent heat transfer is largest in hot, humid
regions. It is least in arid areas with low precipitation
and in high latitudes, where there is little available
energy.
The largest exchange of sensible heat occurs over
tropical deserts, where more than 80 W m
-2
is trans-
ferred to the atmosphere (see Figure 3.30). In contrast
to latent heat, the sensible heat flux is generally small
over the oceans, reaching only 25-40 W m
-2
in areas of
warm currents. Indeed, negative values occur (transfer
to the ocean) where warm continental airmasses move
offshore over cold currents.
the high temperature of the sun (6000 K) (i.e. Wien's
Law). The solar constant has a value of approximately
1368 W m
-2
. The sun and the earth radiate almost as
black bodies (Stefan's Law,
F
=
σ
T
4
), whereas the
atmospheric gases do not. Terrestrial radiation, from
an equivalent black body, amounts to only about 270
W m
-2
due to its low radiating temperature (263 K);
this is infra-red (long-wave) radiation between 4 and
100 µm. Water vapour and carbon dioxide are the
major absorbing gases for infra-red radiation, whereas
the atmosphere is largely transparent to solar radiation
(the greenhouse effect). Trace gas increases are now
augmenting the 'natural' greenhouse effect (33 K). Solar
radiation is lost by reflection, mainly from clouds, and by
absorption (largely by water vapour). The planetary
albedo is 31 per cent; 49 per cent of the extraterrestrial
radiation reaches the surface. The atmosphere is heated
primarily from the surface by the absorption of
terrestrial infra-red radiation and by turbulent heat
transfer. Temperature usually decreases with height at
an average rate of about 6.5°C/km in the troposphere.
In the stratosphere and thermosphere, it increases with
height due to the presence of radiation absorbing gases.
SUMMARY
Almost all energy affecting the earth is derived from solar
radiation, which is of short wavelength (<4 µm) due to
