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