Beaufort scale

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The Beaufort scale is a system for describing wind speed based on observed sea conditions. The scale consists of thirteen forces, ranging from force 0 (calm) to force 12 (hurricane).
It is the standard way to describe wind speed at sea and is used in most nautical weather forecasts.

The Beaufort scale is named after Francis Beaufort, who devised the system in 1805 and later became head of the Hydrographic Office of the British Admiralty.[1]
The original scale defined each force in terms of the effect of the wind on the sails of a man of war. In 1906, with the growth of steam power, the descriptions were changed to describe the state of the sea rather than the sails of a man of war, and at the same time land-based definitions were created.

Approximate wind speed[2] Description[2] Sea conditions[1] Land conditions[1]
knots m s-1
Force 0 < 1 < 0.2 Calm Sea like a mirror. Smoke rises vertically.
Force 1 1 - 3 0.3 - 1.5 Light air Ripples, but no foam crests. Smoke drifts in the wind.
Force 2 4 - 6 1.6 - 3.3 Light breeze Small wavelets. Leaves rustle. Wind felt on face.
Force 3 7 - 10 3.4 - 5.4 Gentle breeze Large wavelets crests, not breaking. Small twigs in constant motion. Light flags extended.
Force 4 11 - 16 5.5 - 7.9 Moderate breeze Numerous whitecaps. Dust, leaves and loose paper raised. Small branches move.
Force 5 17 - 21 8.0 - 10.7 Fresh breeze Many whitecaps, some spray. Small trees sway.
Force 6 22 - 27 10.8 - 13.8 Strong breeze Larger waves form. Whitecaps everywhere. More spray. Large branches move. Whistling in phone wires. Difficult to use umbrellas.
Force 7 28 - 33 13.9 - 17.1 Near gale White foam from breaking waves begins to be blown in streaks. Whole trees in motion.
Force 8 34 - 40 17.2 - 20.7 Gale Edges of wave crests begin to break into spindrift. Twigs break off trees. Difficult to walk.
Force 9 41 - 47 20.8 - 24.4 Severe gale High waves. Sea begins to roll. Spray may reduce visibility. Chimney pots and slates removed.
Force 10 48 - 55 24.5 - 28.4 Storm Very high waves with overhanging crests. Blowing foam gives sea a white appearance. Trees uprooted. Structural damage.
Force 11 56 - 63 28.5 - 32.6 Violent storm Exceptionally high waves. Widespread damage.
Force 12 > 64 > 32.7 Hurricane Air filled with foam. Sea completely white. Visibility greatly reduced. Widespread damage. Very rarely experienced on land.

References

  1. 1.0 1.1 1.2 http://www.bbc.co.uk/weather/features/understanding/beaufort_scale.shtml
  2. 2.0 2.1 http://www.metoffice.gov.uk/weather/marine/guide/beaufortscale.html