Geoscience Reference
In-Depth Information
receipts under clear skies there is at the same time
greater net loss of terrestrial radiation.
Figures 3.29 and 3.30 show the annual vertical
transfers of latent and sensible heat to the
atmosphere. Both fluxes are distributed very
differently over land and seas. Heat expenditure
for evaporation is at a maximum in tropical and
subtropical ocean areas, where it exceeds 160W
m -2 . It is less near the equator, where wind speeds
are somewhat lower and the air has a vapor
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 or moisture.
The largest exchange of sensible heat occurs
over tropical deserts, where more than 80W m -2
is transferred to the atmosphere (see Figure 3.30 ).
In contrast to latent heat, the sensible heat flux is
generally small over the oceans, only reaching
25-40W m -2 in areas of warm currents. Indeed,
negative values occur (transfer to the ocean) where
warm continental air masses move offshore over
cold currents.
Almost all energy affecting the earth is derived from solar radiation, which is of short
wavelength (<4
m) due to the high temperature of the sun (6000K) (i.e., Wien's Law). The
solar constant has a value of approximately 1366W m -2 . The sun and the earth radiate
almost as black bodies (Stefan's Law, F =
μ
σ
T 4 ), whereas the atmospheric gases do not.
Terrestrial radiation, from an equivalent black body, amounts to only about 270W m -2 due
to its low radiating temperature (263K); this is infrared (longwave) radiation between 4 and
100 μ m. Water vapor and carbon dioxide are the major absorbing gases for infrared
radiation, whereas the atmosphere is largely transparent to solar radiation (the greenhouse
effect). Trace-gas increases are now augmenting the 'natural' greenhouse effect (33K). Solar
radiation is lost by reflection, mainly from clouds, and by absorption (largely by water
vapor). The planetary albedo is 31 percent; 49 percent of the extraterrestrial radiation
reaches the surface. The atmosphere is heated primarily from the surface by the absorption
of terrestrial infrared 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.
The excess of net radiation in lower latitudes leads to a poleward energy transport from
tropical latitudes by ocean currents and by the atmosphere. This is in the form of sensible
heat (warm air masses/ocean water) and latent heat (atmospheric water vapor). Air
temperature at any point is affected by the incoming solar radiation and other vertical
energy exchanges, surface properties (slope, albedo, heat capacity), land and sea
distribution and elevation, and also by horizontal advection due to air mass movements
and ocean currents.
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