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.
Description (with effects on open sea)
Calm, sea like a mirror
1 - 3
Light Air; ripples with the appearance of scales formed, but without foam crests.
4 - 6
Light Breeze; small wavelets, crests have a glassy appearance but do not break.
7 - 10
Gentle Breeze; Large wavelets, crests begin to break, foam of glassy appearance, perhaps scattered white horses.
Moderate Breeze; small waves, becoming longer, fairly frequent white horses.
Fresh Breeze; moderate waves taking a more pronounced form, many white horses are formed, chance of some spray.
Strong Breeze; large waves begin to form, the white foam crests are more extensive everywhere, probably some spray.
Near Gale; sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
Gale; moderately high waves of greater length, edges of crests begin to break into spindrift, the foam is blown into well marked streaks along the direction of the wind.
Strong Gale; high waves, dense streaks of foam along the direction of the wind, crests of waves begin to topple tumble and roll over, spray may affect visibility.
Storm; very high waves with long overhanging crests, the resulting foam in great patches is blown in dense white streaks along the direction of the wind, on the whole the sea takes a white appearance, the tumbling of the waves becomes heavy and shocklike, visibility affected.
Violent Storm; exceptionally high waves, small and medium-sized ships may for a time be lost to view, the sea is completely covered with long white patches of foam lying along the direction of the wind, everywhere the crests of the waves are blown into froth, visibility seriously affected.
Hurricane; the air is filled with foam and spray, sea completely white with driving spray, visibility very seriously affected.