Figure 4: Clear sky absorption and scattering of incident solar energy. Values are typically for one air
mass.
20
20
N. Harris, et al., John Wiley & Sons, 1985: Solar Energy Systems Design, New York NY, 774 pp., Out of
print.
10
The atmosphere’s effect on the insolation is different in different places and is dependant on
weather conditions. Some guidelines of how weather affects the insolation follows below
21
.
Cloudy weather:
•
The clouds reflect approximately 40% (can reflect up to 80%)
•
The clouds, water vapour, ice crystals etc. absorb approximately 10% (can be up to
20%)
•
The clouds can give back between 0-50% in diffused insolation to Earth.
Clear weather:
•
Reflection from different particles, water drops and ice crystals is approximately 5%.
•
Absorption in dust and water drops is approximately 15%.
•
The Earth’s surface reflects 0-80% depending on surface (see Albedo below).
5.2.6
Albedo
Some of the incoming radiation from the Sun is reflected and scattered back to the outer
space. Albedo (often with the letter
α)
can have a value from 0 for no reflection to 1 for
complete reflection of light striking the surface. Albedo can be expressed as a percentage
(albedo multiplied by 100) that for some is easier to understand. The radiation reflected can
be calculated with the following equation:
I
refl
=
α∗
I
sc
Often surfaces that are bright have high albedo which means
that they reflect much of the light. With the same reasoning
dark surfaces have low albedo. The radiation that is not
reflected is absorbed. The absorption of sun radiation is
mainly done by the ground, plants and oceans but some is
instead absorbed by the atmosphere and the clouds. UV light is
mainly absorbed by oxygen and ozone in the stratosphere.
Earth has as a mean value of 0.3 in albedo. Some other
surface’s albedo are shown in table 2
22
.
Clouds Albedo
Clouds have a great effect on the albedo of the planets. Clouds reflect much more light back
then blue sky does. The albedo of a cloud depends on several factors, including the height,
size, and the number and size of droplets inside the cloud. If the cloud contains many big
droplets and therefore has a large total surface that reflects, the albedo is high. You can see
this when you are under a big cumulonimbus cloud, where not much of the sunlight passes
through. A cumulonimbus cloud has an albedo of up to 0.9 and looks black from the ground
but light from space. Stratus clouds have an albedo of 0.4 - 0.65 and cirrus clouds have
around 0.2 - 0.4.
23
21
Petterson F, 1985: Solenergi Teori, Forskning & Praktisk användbarhet, Stockholm: Ingenjörsförlaget AB
22
University of Leeds:
, 2005-03-23
23
ESPERE-ENC;
, 2005-03-23
Table 2: Albedo for different
terrains.
Surface
Albedo
Fresh snow
0.85-0.95
Dry sand
0.35-0.40
Plains and
farmlands
0.20-0.30
Tropical forest ~0.13
Ocean
0.04-0.10
