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The Beaufort Wind Scale

The Beaufort Scale, used throughout the marine world, has developed over many years since it was first devised by Admiral Francis Beaufort in 1806. Today, the Beaufort scale is defined for seamen in terms of sea state.  It's emphasis is more on the observed effect of the wind, rather than the actual wind speed.

Beaufort Force	Wind Speed Knots	Description	Sea Condition
0	0	Calm	Sea like a mirror
1	1 - 3	Light air	Ripples but without foam crests
2	4 - 6	Light breeze	Small wavelets. Crests do not break
3	7 - 10	Gentle breeze	Large wavelets. Perhaps scattered white horses
4	11 - 16	Moderate breeze	Small waves. Fairly frequent white horses
5	17 - 21	Fresh breeze	Moderate waves. Many white horses
6	22 - 27	Strong breeze	Large waves begin to form; white foam crests. Probably spray
7	28 - 33	Near gale	Sea heaps up and white foam blown in streaks along the direction of the wind
8	34 - 40	Gale	Moderately high waves. Crests begin to break into spindrift. The foam is blown in well marked streaks along the direction of the wind
9	41 - 47	Severe gale	High waves. Dense foam along the direction of the wind. Crests of waves begin to roll over. Spray may affect visibility
10	48 - 55	Storm	Very high waves with long overhanging crests. The surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shock-like. Visibility affected
11	56 - 63	Violent storm	Exceptionally high waves. The sea is completely covered with long white patches of foam lying in the direction of the wind. Visibility affected
12	64+	Hurricane	The air is filled with foam and spray. Sea completely white with driving spray. Visibility very seriously affected