The Beaufort Scale - free nautical information from idealspain.com

Force	Mean Speed(knots)	Description	State of sea	Ashore	Probable Wave Height (m)	Probable Max Wave Height
0	0-1	Calm	Flat	Smoke rises vertically	0	0
1	1-3	Light air	Ripples	Smoke shows wind direction	0.1	0
2	4-6	Light breeze	Small wavelets, non breaking	Leaves rustle	0.2	0.3
3	7-10	Gentle breeze	Large wavelets, crests breaking	Wind extends small flag, leaves in motion	0.6	1
4	11-16	Moderate breeze	Fairly frequent white horses	Small branches begin to move	1	1.5
5	17-21	Fresh breeze	Moderate waves, many white horses, some spray	Small trees begin to sway	2	2.5
6	22-27	Strong breeze	Large waves, white foam crests, some spray	Large branches move	3	4
7	28-33	Near gale	White foam blows from crests	Branches move, difficult to walk against the wind	4	5.5
8	34-40	Gale	Mod high waves, foam in streaks along surface	Twigs break off trees	5.5	7.5
9	41-47	Strong gale	High waves, tumbling crests, dense streaks of foam, much spray	Slates, chimneys begin to be moved	7	10
10	48-55	Storm	Very high waves with long overhanging crests, heavy tumbling sea	Trees uprooted, structural damage to buildings	9	12.5
11	56-63	Violent storm	Sea completely covered with long white foam patches	Wide spread damage to buildings etc	11.5	16
12	64 +	Hurricane	Air filled with foam and driving spray. Visibility very seriously affected	As above	14	Above 16