- In 1805 Admiral Beaufort drew up a scale of wind strengths related to commonly observable phenomena.
|Force||Description||Commonly Observable Phenomena||Wind speed Km/h||Wind speed Knots|
|0||Calm||Smoke rises vertically||0-1||<1|
|1||Light air||Wind direction indicated by smoke but not wind vanes||1-5||1-3|
|2||Light breeze||Wind noticeable, leaves move, wind vane moves.||6-11||4-6|
|3||Gentle breeze||Leaves and small twigs in constant motion.||12-19||7-10|
|4||Moderate breeze||Wind raises dust and loose paper. Small branches are moved.||20-29||11-16|
|5||Fresh breeze||Small trees in leaf start to sway.Crested wavelets form on inland waters.||30-39||17-21|
|6||Strong breeze||Large branches in motion, wind whistles. Umbrellas used with difficulty.||40-50||22-27|
|7||Near gale.||Trees in motion, awkward to walk against wind.||51-61||28-33|
|8||Gale||Twigs break, hard to walk.||62-74||34-40|
|9||Strong gale||Some structural damage may occur, slates removed etc.||75-87||41-47|
|10||Storm||Trees uprooted, considerable structural damage.||88-101||48-55|
|11||Violent storm||Widespread damage.||102-117||56-63|
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