Environmental Engineering Reference
In-Depth Information
During the night hours, until 6 o'clock in the morning, the wind velocity is at its
minimum. Thereafter the velocity of the air movement increases slowly. At 9
o'clock in the morning the diurnal mean value is normally reached. In the early
afternoon, between 2 and 4 o'clock, the wind speed reaches its maximum and then
decreases again. Between 7 and 8 o'clock in the afternoon it again reaches its
diurnal mean value and at midnight or afterwards its diurnal minimum.
7.5
7.5
Inversion type
7.0
7.0
Maximum value
Standard deviation
6.5
6.5
Mean value
Standard deviation
6.0
6.0
Minimum value
5.5
5.5
5.0
5.0
Maximum value
Standard deviation
4.5
4.5
Mean value
Standard deviation
4.0
4.0
Minimum value
3.5
3.5
Ground type
3.0
3.0
0 2 4 6 8 1012141618202224
0 2 4 6 8 1012141618202224
Fig. 2.34
Mean values of mean diurnal wind velocities at different sites (ground type: data
from Norderney/Germany; inversion type: data from Hohenpeißenberg/Germany; data
according to /2-12/)
Time in hours
Time in hours
On some days, the diurnal course of the wind velocity can deviate significantly
from the mean diurnal course given in the yearly average due to changing local
weather conditions. However, in the course of one year, stationary weather condi-
tions generally prevail. The diurnal course exemplarily described for the site of
Norderney/Germany is equal to the diurnal course at a thermal stratification dur-
ing robust weather conditions, with a strong mixing of the air layer close to the
ground during the day, due to the incident solar radiation and a stable stratifica-
tion during the night /2-15/.
At exposed mountain top sites (e.g. Hohenpeißenberg/Germany, Fig. 2.34) and
less topographically structured areas, between 50 to 100 m above ground, the
diurnal course of the wind velocity is reversed compared to the ground type. It is
referenced as the inversion type. The velocity maximum is reached during the
night, the minimum at mid-day or in the afternoon. The reversal of the mean diur-
nal course of wind velocity can be explained with the varying thermal stratifica-
tion between day and night. During the day, with an unstable thermal stratification
due to the insulated solar energy and the reflected thermal radiation, the planetary
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