BEAUFORT scale

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Wind speed Terms Speed in knots Speed in km/h Sea state
0 Calm Less than 1 Less than 1 Mirror
1 Very light breeze 1 to 3 1 to 5 A few ripples
2 Light breeze 4 to 6 6 to 11 Non-breaking wavelets
3 Gentle breeze 7 to 10 12 to 19 Appearance of whitecaps
4 Moderate breeze 11 to 15 20 to 28 Small waves, many whitecaps
5 Fresh breeze 16 to 21 29 to 38 Moderate waves, whitecaps, some spray.
6 Strong breeze 22 to 27 39 to 49 White foam tips, spray.
7 Near gale 28 to 33 50 to 61 Waves beginning to break, trails of foam and a lot of spray.
8 Gale 34 to 40 62 to 74 Breakers, swirls of foam at the tip of the waves, a lot of spray.
9 Strong gale 41 to 47 75 to 88 High breakers. A lot of swirling foam. Visibiility reduced by spray.
10 Storm 48 to 55 89 to 102 Large breakers. Large sheets of foam. Visibiility reduced by spray.
11 Violent storm 56 to 63 103 to 117 Very large breakers. Very large sheets of foam. Visibility greatly reduced by spray.
12 Hurricane equal to or greater than 64 equal to or greater than 118 Enormous breakers. The sea is completely white. Visibility substantially reduced by spray.
This scale was thought up by the British Admiral Francis Beaufort (1774-1857) in 1805.
  Cyclone, hurricane, typhoon : These terms are only used for tropical cyclones
  Wind is the horizontal movement of air and is characterised by the measurement of two magnitudes : that of its direction, (i.e. where it is coming from ) expressed in degrees, and that of its speed expressed in metres per second or in knots (nautical miles per hour, 1 nautical mile = 1852 metres = 1 minute of latitude ); when simultaneous values for both magnitudes are measured at a given point, this is also said to be the wind and in fact defines the horizontal component of the air movement speed compared with the Earth. The wind direction is measured at ground level using a wind vane, and is the angular direction of where the wind is coming from in relation to the cardinal directions; The wind speed is measured at ground level using an anemometer and is often calculated (especially at sea) inside concrete variation intervals that give an order of magnitude referred to as wind speed.