Environmental Engineering Reference
In-Depth Information
400
1. 6
A
350
1. 4
300
1. 2
250
1
UV-B
200
0.8
15 0
0.6
PAR
10 0
UV-A
0.4
50
0.2
0
0
06:00
08:00
10:00
12:00
14:00
16:00
18:00
20:00
Local time [h]
Figure 3 . ELDONET measurements at Ljubljana (Slovenia) during the solar eclipse on
11 August 1999.
Erlangen (Germany, 50° N, 11° E) represents a typical central European location.
The monthly doses peak at about 244 MJ m -2 for PAR in the summer. July had more
cloud cover than June or August. Also UV-A and UV-B irradiances were maximal in
June which corresponds with the highest solar angles. The same is true for the ratio UV-
B/UV-A while the ratios UV-B/PAR and UV-A/PAR have their maximum in July.
Plotting the UV-B/PAR ratios on a daily basis also shows a clear maximum in July
which is due to the antagonistic functions of the decreasing total ozone column values
and the decreasing solar angles in autumn.
The northernmost station of the network (Abisko, North Sweden, 69° N, 19° E)
is located above the Nordic polar circle and also has maximal irradiances in June.
During winter the sun does not rise above the horizon resulting in extremely low
irradiances. In summer the PAR peak doses exceed those measured in Erlangen
corresponding to the fact that in this month the sun does not sink below the horizon so
that the doses are effectively accumulated over 24 h periods. While UV-B/PAR and
UV-B/UV-A ratios peak in July, the UV-A/PAR has a maximum in December and
January which is due to the higher amount of diffuse skylight during the winter months
in comparison to the direct radiation which increases with longer wavelengths.
Figure 4 summarizes the yearly doses for 1998 for all European stations at which
measurements were performed on more than 250 days of the year arranged in a
North-South gradient. While there is a clear dependence on the latitude one can detect a
few remarkable exceptions. Helgoland (island in the German bight, 54° N, 8° E) shows
comparatively higher doses than Rostock with a comparable latitude (on the Southern
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