The Beaufort Scale

The Beaufort Scale of Wind Force was developed in 1805 by Admiral Sir Francis Beaufort as a means for sailors to gauge wind speeds through visual observations of the sea state. Land based equivalent observations were added later on. The scale runs from force 0 (calm) to force 12 (Hurricane). The Met. Office issue Gale Warnings for sea areas when winds of force 8 or greater are forecast, and, locally, strong wind warnings may be issued for winds of force 5 to 7.
The Beaufort Scale: Sea based specification
Force Speed
(10 m above ground) 		Description Specifications for use on land
MPH Knots
0 0-1 0-1 Calm Sea like a mirror
1 1-3 1-3 Light air Ripple with the appearance of scales are formed, but without foam crests.
2 4-7 4-6 Light Breeze Small wavelets, still short, but more pronounced. Crests have a glassy appearance and do not break.
3 8-12 7-10 Gentle Breeze Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses.
4 13-18 11-16 Moderate Breeze Small waves, becoming larger; fairly frequent white horses.
5 19-24 17-21 Fresh Breeze Moderate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray.
6 25-31 22-27 Strong Breeze Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray.
7 32-38 28-33 Near Gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
8 39-46 34-40 Gale Moderately high waves of greater length; edges of crests begin to break into spindrift. The foam is blown in well-marked streaks along the direction of the wind.
9 47-54 41-47 Severe 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.
10 55-63 48-55 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 surface of the sea takes on a white appearance. The 'tumbling' of the sea becomes heavy and shock-like. Visibility affected.
11 64-72 56-63 Violent Storm Exceptionally high waves (small and medium-size ships might be for a time lost to view behind the waves). The sea is completely covered with long white patches of foam lying along the direction of the wind. Everywhere the edges of the wave crests are blown into froth. Visibility affected.
12 73-83 64-71 Hurricane The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected.