Environmental Engineering Reference
In-Depth Information
Table 5.1
Wind Speed Classification of the Beaufort Wind Scale
Bf
v in m/s
Description
Effects
0
0-0.2
Calm
Smoke rises vertically
1
0.3-1.5
Light air
Smoke moves slightly and shows direction
of wind
2
1.6-3.3
Light breeze
Wind can be felt. Leaves start to rustle
3
3.4-5.4
Gentle breeze
Small branches start to sway. Wind
extends light flags
4
5.5-7.9
Moderate breeze
Larger branches sway. Loose dust on
ground moves
5
8.0-10.7
Fresh breeze
Small trees sway
6
10.8-13.8
Strong breeze
Trees begin to bend, whistling in wires
7
13.9-17.1
Moderate gale
Large trees sway
8
17.2-20.7
Fresh gale
Twigs break from trees
9
20.8-24.4
Strong gale
Branches break from trees, minor damage
to buildings
10
24.5-28.4
Full gale/storm
Trees are uprooted
11
28.5-32.6
Violent storm
Widespread damage
12
32.7
Hurricane
Structural damage
Note:
Bf, Beaufort force; v, wind speed in m/s (1 m/s = 3.6 km/h = 2.24 mph)
Definition of wind force
In meteorology the
Beaufort scale
is often applied to give the wind force. This
scale allows an approximate estimation of wind speed without complicated
measurement systems; however, this scale is less useful for technical purposes.
Therefore, the wind speed
v
given in the SI unit m/s is used instead. Table 5.1
compares the Beaufort classes with the corresponding wind speed values.
Wind speed distributions
Wind speed distributions are commonly used to indicate the annual available
wind energy. These distributions are estimated using measurements, wind maps
or computer analysis. Tables or statistical functions can give the distribution.
Figure 5.1 shows the
relative distribution h
(
v
) of wind speed
v
in
Karlsruhe, which is located in south Germany near the Black Forest. This
distribution indicates how often a certain wind speed occurs. It is immediately
obvious that the wind energy potential in Karlsruhe is very low. The sum of
frequencies with wind speeds lower than 4 m/s is more than 70 per cent. In
other words, for practical wind generator use, wind speeds above 4 m/s exist
for only 30 per cent of the time.
The wind speed measurement interval can cause uncertainties in the
estimation of wind speed frequency distributions. If the average of the wind
