Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 46

on the northern hemisphere to be significantly lower during the winter than during
the summer. Additionally, north of the equator the time spans when the sun shines
throughout the course of one day, are shorter than in the summer. This becomes
clear in Fig. 2.18; the left side of the graphic shows the mean hourly solar position
above the horizon at a monthly average in Central Europe. For the winter months,
the short period of time when the sun is above the horizon, as well as, the very
low solar position above the horizon compared to the summer become apparent.
This changes during the summer, as the northern hemisphere is facing the sun.
The reverse is true for the southern hemisphere.

12

12

24 8

1

1 0

1

Fig. 2.18 Monthly mean solar position (on the left) and monthly mean degree of cloudi-
ness (on the right) exemplarily for one site in South Germany (for data see /2-12/)

In a first approximation, the weakening of solar radiation within the atmos-
phere of the earth is proportional to the radiation course through the atmosphere.
During the summer a larger part of the radiation incident at the top atmospheric
rim thus reaches the earth surface at most places of the globe. This is due to the
higher solar position, and thus, the on-average shorter radiation path. Addition-
ally, the radiation absorption and the reflection in the atmosphere depend on the
water content of the atmosphere, and thus, on how overcast the sky is. Due to
predominant meteorological conditions in many parts of the earth, they are subject
to significant seasonal fluctuations. Fig. 2.18 (on the right) therefore shows the
likelihood of the occurrence of certain cloud amounts on a monthly average, ex-
emplarily for a South-German site. The cloud amounts which are a meteorological
measure for the mean hourly cloudiness, fluctuate between 0 (no clouds) and 8
(completely overcast). When comparing the likelihood of occurrence shown in
Fig. 2.18 (on the right) it becomes apparent that the atmosphere at the investigated
spot is on average decidedly cloudier during the winter months than during the
summer. This statement can generally be applied to other sites and years through-
out the wide parts of the northern and southern hemisphere. Altogether, shorter
periods of sunshine, as well as, small radiation incident angles, an over-
proportional cloudiness, and thus a weakening of radiation in the atmosphere
characterise the winter in comparison to the summer.