Beaufort Scale

In 1805 Admiral Beaufort drew up a scale of wind strengths related to commonly observable phenomena.
ForceDescriptionCommonly Observable PhenomenaWind speed Km/hWind speed Knots
0CalmSmoke rises vertically0-1<1
1Light airWind direction indicated by smoke but not wind vanes1-51-3
2Light breeze Wind noticeable, leaves move, wind vane moves.6-114-6
3Gentle breezeLeaves and small twigs in constant motion.12-197-10
4Moderate breezeWind raises dust and loose paper. Small branches are moved.20-2911-16
5Fresh breezeSmall trees in leaf start to sway.Crested wavelets form on inland waters.30-3917-21
6Strong breezeLarge branches in motion, wind whistles. Umbrellas used with difficulty.40-5022-27
7Near gale.Trees in motion, awkward to walk against wind.51-6128-33
8GaleTwigs break, hard to walk.62-7434-40
9Strong galeSome structural damage may occur, slates removed etc.75-8741-47
10StormTrees uprooted, considerable structural damage.88-10148-55
11Violent stormWidespread damage.102-11756-63
12HurricaneWidespread damage.118+>63

The Beaufort scale numbers come rather close to being a unit of measurement, because they are equal to the whole number closest to 0.66 times the wind velocity in miles per hour raised to the exponent 2/3.

Beaufort force = 0.66 x velocity2/3 where velocity is in mph

See also: Fujita-Pearson Scale, Wind Speed, Wind Velocity.

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Subjects: Meteorology