Beaufort wind scale

learnthesea.com

The scale forecasters use. Speeds are the mean over ten minutes; gusts run higher. For most cruisers F4 is a good sail, F6 is hard work, F8 is a gale to be elsewhere for.

F	Knots	Name	Sea state	On the water
0	<1	Calm	Mirror	Smoke rises vertically
1	1–3	Light air	Ripples	Smoke drifts; barely steerage
2	4–6	Light breeze	Small wavelets	Wind felt on the face; sails fill
3	7–10	Gentle breeze	Large wavelets, a few crests	Pleasant sailing, slight heel
4	11–16	Moderate breeze	Small waves, frequent white horses	Lively sailing — the cruiser’s sweet spot
5	17–21	Fresh breeze	Moderate waves, many white horses	Time to think about a reef
6	22–27	Strong breeze	Large waves, white foam crests	The “yachtsman’s gale” — reefed, hard work
7	28–33	Near gale	Sea heaps up, foam blows in streaks	Uncomfortable; consider shelter
8	34–40	Gale	Moderately high waves, foam streaks	Deep-reefed; most cruisers should be in
9	41–47	Severe gale	High waves, crests topple, spray	Serious; survival sailing
10	48–55	Storm	Very high waves, sea white with foam	Rare inshore; dangerous
11	56–63	Violent storm	Exceptionally high waves	Rarely experienced
12	64+	Hurricane force	Air filled with foam & spray	—