Beaufort wind scale

Force	Term	Knots	Sea description
0	Calm	< 1	Sea like a mirror.
1	Light air	1–3	Ripples with the appearance of scales, without foam crests.
2	Light breeze	4–6	Small wavelets, crests glassy, not breaking.
3	Gentle breeze	7–10	Large wavelets, crests begin to break, scattered whitecaps.
4	Moderate breeze	11–16	Small waves, becoming longer; fairly frequent whitecaps.
5	Fresh breeze	17–21	Moderate waves, many whitecaps, some spray.
6	Strong breeze	22–27	Large waves begin to form, white foam crests extensive, some airborne spray.
7	Near gale	28–33	Sea heaps up, white foam from breaking waves blown in streaks.
8	Gale	34–40	Moderately high waves of greater length; edges of crests break into spindrift.
9	Strong gale	41–47	High waves; dense streaks of foam; crests begin to roll over; spray may reduce visibility.
10	Storm	48–55	Very high waves with long overhanging crests; sea takes white appearance; heavy tumbling.
11	Violent storm	56–63	Exceptionally high waves; sea completely covered with long white patches of foam.
12	Hurricane	≥ 64	Air filled with foam and spray; sea completely white with driving spray; visibility very seriously affected.