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Reference

Beaufort Scale

Wind turbines start generating electricity at 3.5 metres per second, equivalent to 3 on the Beaufort scale, and will shut down automatically at wind speeds above 25 metres per second or gale force 10 to protect themselves from damage.

Critics of renewable energy often refer to the automatic shut down of turbines negatively; the fact that turbines only do so at wind speeds which uproots trees may rather be seen as eminently sensible!

The power output of wind turbine is a function of the cube of the wind speed, so even the smallest increase in wind speed results in significantly higher electricity production.

  Miles per hour Metres per second Description Features
0 0-1 0-0.45 Calm Smoke rises vertically.
1 1-3 0.45-1.3 Light air Direction of wind shown by smoke drift, but not by wind vanes.
2 4-7 1.8-3.1 Light Breeze Wind felt on face; leaves rustle; ordinary vanes moved by wind.
3 8-12 3.6-5.4 Gentle Breeze Leaves and small twigs in constant motion; wind extends light flag.
4 13-18 5.8-8.0 Moderate Breeze Raises dust and loose paper; small branches are moved.
5 19-24 8.5-10.7 Fresh Breeze Small trees in leaf begin to sway; crested wavelets form on inland waters.
6 25-31 11.2-13.9 Strong Breeze Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty.
7 32-38 14.3-17.0 Near Gale Whole trees in motion; inconvenience felt when walking against the wind.
8 39-46 17.4-20.6 Gale Breaks twigs off trees; generally impedes progress.
9 47-54 21.0-24.1 Severe Gale Slight structural damage occurs (chimney-pots and slates removed).
10 55-63 24.5-28.2 Storm Seldom experienced inland; trees uprooted; considerable structural damage occurs.
11 64-72 28.6-32.2 Violent Storm Very rarely experienced; accompanied by wide-spread damage.
12 73-83 32.6-37.1 Hurricane  

Devised by Sir Francis Beaufort in 1805, this is a system for estimating wind strengths without the use of instruments, based on the effects wind has on the physical environment, ranging from 0 for calm through to 12 for hurricane.

The cells shown in blue denote the operational range of a typical wind turbines installed in the UK. The wind speed is higher at the top of turbine towers, however, so it might not seem quite as windy at ground level.