Environmental Engineering Reference
In-Depth Information
Spectral irradiance (W/m
2
µ
m)
2500
ultraviolet
visible
infrared
2000
1500
spectrum AM0
(extraterrestrial)
1000
spectrum AM1.5
(terrestrial)
500
0
0
0.5
1.0
1.5
2.0
2.5
3.0
Wavelength ( m)
µ
Note: AM
0 is the extraterrestrial spectrum;
AM
1.5 is the spectrum on the Earth's surface at a sun
height of 41.8°
Figure 2.3
Spectrum of Sunlight
The relationship between the sun height
γ
S
and the air mass (AM) is:
1
sin
AM
=
(2.10)
S
The AM value is a unitless measure of the length of the path of light through
the atmosphere; it is expressed in multiples of the thickness of atmosphere. If
the sun is at its zenith, AM is equal to 1, i.e. the light is passing vertically
through the atmosphere. The AM value outside the atmosphere is zero. Figure
2.4 shows the highest position of the sun at solar noon and the corresponding
AM values for various days of a year for Berlin and Cairo.
The elevation of the sun also influences the irradiation received at the
surface of the Earth, which is thus dependent on the time of the year. Clouds
Table 2.4
Reduction Influences at Different Sun Heights
Sun Height
Air
Absorption
Rayleigh
Mie
Total
(
γ
S
)
Mass
(%)
scattering
scattering
reduction
(AM)
(%)
(%)
(%)
90°
1.00
8.7
9.4
0-25.6
17.3-38.5
60°
1.15
9.2
10.5
0.7-29.5
19.4-42.8
30°
2.00
11.2
16.3
4.1-44.9
28.8-59.1
10°
5.76
16.2
31.9
15.4-74.3
51.8-85.4
5°
11.5
19.5
42.5
24.6-86.5
65.1-93.8
Source:
according to Schulze, 1970
