The Beaufort Scale

Force	Speed		Conditions
	knots	km/h
0	<1	<1	Calm, sea like a mirror.
1	1-3	1-5	Light air, ripples that look like fish scales are formed but without foam crests. Crests look glassy and do not break.
2	4-6	6-11	Light breeze, small wavelets (0.2m), still short but more pronounced. Crests have a glassy appearance.
3	7-10	12-19	Gentle breeze, large wavelets (0.6m), crests begin to break. Glassy-looking foam. Perhaps scattered white horses.
4	11-16	20-29	Moderate breeze, small waves (1m), becoming longer, some white horses.
5	17-21	30-39	Fresh breeze, moderate waves (1.8m), many white horses.
6	22-27	40-50	Strong breeze, large waves (3m), probably some spray.
7	28-33	51-61	Near gale, mounting sea (4m) with foam blown in streaks downwind.
8	34-40	62-74	Gale, moderately high waves (5.5m), crests break into spindrift.
9	41-47	76-87	Strong gale, high waves (7m), dense foam, visibility affected.
10	48-55	88-102	Storm, very high waves (9m), heavy sea roll, visibility impaired. Surface generally white.
11	56-63	103-118	Violent storm, exceptionally high waves (11m), visibility poor.
12	64+	119	Hurricane, 14m waves, air filled with foam and spray, visibility bad.

Fujita Scale for Tornado Intensity F-Scale Winds Damage F0 (weak) 40-72 mph Light Damage F1 (weak) 73-112 mph Moderate Damage F2 (strong) 113-157 mph Considerable Damage F3 (strong) 158-206 mph Severe Damage F4 (violent) 207-260 mph Devastating Damage F5 (violent) 261-318 mph Incredible Damage

Wave heights quoted are approximately those that may be expected in the open sea. In enclosed waters the waves will be smaller and steeper. Fetch, depth, swell, heavy rain and tide will also affect their height, and there will also usually be a time lag between any increase in the wind and the consequent increase in the sea.