Geoscience Reference
In-Depth Information
3.2.1.1.
Solar radiation
The net solar radiation (short waves) is the sum of the incident
solar radiation diffused by the gases and particles, in the atmosphere,
minus the reflected radiation, which is function of the surface albedo.
This albedo (reflecting power) ranges between 0 and 1, from a black
body to a perfectly reflective mirror. Snow, sea ice and sand have high
albedos (in the order of 0.8), whereas water has a low albedo
(approximately 0.1), so that visible light, above all at short
wavelengths, penetrates into the upper layers.
On average, the solar flux value is 1
,
365 Wm
2
before it enters the
atmosphere on a disk perpendicular to the radiation direction. But
because the Earth is round, the flux received effectively depends on
the latitude. It also varies with the season. Diffusion through the
atmosphere (which gives it its blue color) leads to a significant loss of
this radiation, since some of the solar flux is sent back into space.
Clouds also diffuse and reflect a more or less significant part of the
solar radiation into space.
3.2.1.2.
Infrared radiation
Net infrared radiation (long waves) denotes the difference between
the radiation emitted downward by the atmosphere's components and
the Sun, and the emission from the surface, which depends on its
emissivity (between 0, a perfectly reflecting body, and 1, a black
body). The sea has a high emissivity so it is an emitter of infrared
radiation.
The spectral absorption of the atmosphere is a function of its
components. Absorption in the atmosphere is mainly due to water
vapor, carbon dioxide, methane, ozone, and numerous trace gases that
are present in small quantities in the atmosphere. For any gas,
concentration determines its absorption in one or more specific
spectral bands. Water vapor is the most abundant component, and its
spectral absorption domain is also very broad (many spectral bands).
The clouds also absorb infrared radiation at all wavelengths.
