Environmental Engineering Reference
In-Depth Information
Table 8.1
The Beaufort wind scale
.
bft
v
in m/s
Name
Description
0
0-0.2
Calm
Smoke rises vertically
1
0.3
−
1.5
Light air
Wind direction only detectable by smoke
2
1.6-3.3
Light breeze
Wind noticeable, leaves rustle
3
3.4-5.4
Gentle breeze
Leaves and small twigs in constant motion
4
5.5-7.9
Moderate breeze Wind moves branches and thin twigs, raises dust
5
8.0-10.7 Fresh wind
Small trees begin to sway
6
10.8-13.8 Strong wind
Large branches in motion, overhead wires whistle
7
13.9-17.1 Moderate gale
Whole trees in motion, noticeable diffi culty in
walking
8
17.2
−
20.7 Fresh gale
Wind breaks branches off trees
9
20.8
−
24.4 Strong gale
Minor structural damage
10
24.5
−
28.4 Heavy storm
Wind uproots trees
11
28.5-32.6 Violent storm
Heavy storm damage
12
32.7 Hurricane
Devastation occurs
≥
10 m/s = 36 km/h = 22.4 mph.
Systems that use wind energy must cope with a strong fl uctuation in wind supply.
On the one hand, they have to use the energy supplied by the wind even when wind
speeds are low; on the other hand, they should suffer no damage even when wind
speeds are extreme. Therefore, most wind turbines move into storm mode when
wind velocity is very high.
Wind Speed Records
The strongest gust of wind measured to date had a wind velocity of 412 kilo-
metres per hour (114 metres per second, 256 mph) on 12 April 1934 on Mount
Washington in the USA. The highest ten-minute average of 372 kilometres per hour
(231 mph) was recorded on the same day on Mount Washington. Tornados reach even
higher wind speeds. Radar has established wind speeds of around 500 kilometres per
hour (139 metres per second, 311 mph) with the strongest tornados recorded thus far. At
this rate the wind capacity per square metre totals more than 1.6 megawatts or over 2000
horsepower. This same effect would be achieved if four large lorries of 500 horsepower
each were driven at full speed towards a one square metre surface. On this basis it is not
diffi cult to understand the extremely destructive effect of tornados.
Modern wind turbines use part of the kinetic energy of the wind. In so doing, they
slow down the wind. In principle, it is not possible to use all the wind's capacity.
This would require the wind to be stopped completely, and in this case the wind
stream would come to a standstill. This was something the German physicist Albert
