The Beaufort scale, force 0 to 12.

Wind force from 0 to 12: the speed at 10 m, what the open sea looks like and the probable wave height. Near the coast waves are shorter and steeper.

Force	Name	Knots	km/h	mph	Waves	The sea
0	Calm	< 1	< 1	< 1		Like a mirror.
1	Light air	1–3	1–5	1–3	0.1 m	Ripples, no crests.
2	Light breeze	4–6	6–11	4–7	0.2 m	Small wavelets; the crests don't break.
3	Gentle breeze	7–10	12–19	8–12	0.6 m	Large wavelets, crests begin to break, a few whitecaps.
4	Moderate breeze	11–16	20–28	13–18	1 m	Small waves growing longer, frequent whitecaps.
5	Fresh breeze	17–21	29–38	19–24	2 m	Moderate, longer waves, many whitecaps, some spray.
6	Strong breeze	22–27	39–49	25–31	3 m	Large waves, white foam crests everywhere, spray.
7	Near gale	28–33	50–61	32–38	4 m	The sea heaps up; foam is blown in streaks.
8	Gale	34–40	62–74	39–46	5.5 m	Moderately high, longer waves; crests break into spindrift.
9	Strong gale	41–47	75–88	47–54	7 m	High waves, dense streaks of foam, spray cuts visibility.
10	Storm	48–55	89–102	55–63	9 m	Very high waves with overhanging crests; the sea looks white.
11	Violent storm	56–63	103–117	64–72	11.5 m	Exceptionally high waves; visibility badly cut.
12	Hurricane force	≥ 64	≥ 118	≥ 73	14 m	The air is full of foam and spray; the sea completely white.

The instrument beside the helm.