The Beaufort wind scale is a standard scale, running from force 0 for calm to force 12 hurricane and above for the description of wind speed. Each value represents a specific range and classification of wind speeds with accompanying descriptions of the effects on surface features. It was originally developed as a system for estimating wind strengths without the use of instruments.
It was introduced in 1806 by Admiral Sir Francis Beaufort (1774-1857) of the British navy to describe wind effects on a fully rigged man-of-war frigate of the period, and it was later modified to include descriptions of effects on land features as well. It is currently still in use for this same purpose as well as to tie together various components of weather (wind strength, sea state, observable effects) into a unified picture.
| The Beaufort Scale (for use at sea) | ||||||
| FORCE | DESCRIPTION | SEA STATE | SPEED | |||
| knots | m/s | |||||
| 0 | calm | like a mirror | <1 | 0.0-0.2 | ||
| 1 | light air | ripples, no foam | 1-3 | 0.3-1.5 | ||
| 2 | light breeze | small wavelets, smooth crests with glassy appearance | 4-6 | 1.6-3.3 | ||
| 3 | gentle breeze | large wavelets, some crests break, some white horses | 7-10 | 3.4-5.4 | ||
| 4 | moderate breeze | small waves, frequent white horses | 11-16 | 5.5-7.9 | ||
| 5 | fresh breeze | moderate rather long waves, many white horses, some spray | 17-21 | 8.0-10.7 | ||
| 6 | strong breeze | some large waves, extensive white foam crests, some spray | 22-27 | 10.8-13.8 | ||
| 7 | near gale | sea heaped up, streaks of foam blowing with the wind | 28-33 | 13.9-17.1 | ||
| 8 | gale | fairly high and long waves, crests breaking into spindrift, blowing foam in prominent streaks | 34-40 | 17.2-20.7 | ||
| 9 | strong gale | high waves, dense foam streaks in wind, wave-crests topple and roll over, spray reduces visibility | 41-47 | 20.8-24.4 | ||
| 10 | storm | very high waves, overhanging crests, dense blowing foam, heavy tumbling sea appears white, visibility poor | 48-55 | 24.5-28.4 | ||
| 11 | violent storm | exceptionally high waves, hiding small ships, sea covered with foam, crests blown into froth, visibility poor | 56-63 | 28.5-32.6 | ||
| 12 | hurricane | air filled with foam and spray, sea white, visibility extremely bad | >64 | 32.7 | ||
