Environmental Engineering Reference
In-Depth Information
Figure 2.2.2 The earth's energy balance
The numbers give the annual energy fl ow per square meter, averaged over the entire
earth for the period from March 2000 to May 2004. We see that ~30% of the 341 W/m 2
of incoming radiation is refl ected and is not absorbed by the earth's surface. The broad
arrows indicate schematically the fl ow of different types of energy in proportion to their
relative magnitudes. In the formation of clouds, energy in the form of heat is released
and this is known as the latent heat of condensation. Figure reproduced with permission
from Trenberth et al. [2.1]. Copyright (2009), American Meteorological Society .
(small particles, such as dust caused by volcano eruptions) in the atmos-
phere, and part of it gets refl ected by the surface of the earth (in particu-
lar, white surfaces such as snow, ice, or deserts). This “ albedo effect
plus the atmospheric refl ections are responsible for refl ecting 30% of the
incoming energy. If we correct for this effect and conduct our energy bal-
ance with Stefan-Boltzmann's Law, we fi nd that the effective tempera-
ture of the earth would be
19ºC, which is too low for water to exist on
the surface of the earth (see Figure 2.2.3 ). The difference can be
explained if we look more closely at the fate of the radiation emitted by
the earth. In our
19ºC estimate, we assumed all the radiation emanating
from the earth is being emitted into space. The atmosphere, however,
refl ects back a signifi cant part of this energy. This refl ection is called the
Search WWH ::




Custom Search