Beaufort wind scale
 

Beaufort Number	Wind Velocity (knots)	Seaman’s Term	Sea Condition	Typical** Wave Height Feet	Typical*** Condition Attainment Time
0	0-1	Calm	Glassy-smooth, mirror-like	Smooth	- -
1	2-3	Light air	Scale-like ripples	Ripples	1-10 min
2	4-6	Light breeze	Small, short wavelets with glassy crests	1/3	5-15 min
3	7-10	Gentle breeze	Large wavelets, crests begin to break, occasional form	1-2	5-20 min
4	11-16	Moderate breeze	Small waves, some whitecaps, more frequent form	2-3	15-60 min
5	17-21	Fresh breeze	Moderate longer waves, better formed, many whitecaps, much foam, some spray	3-4	15-60 min
6	22-27	Strong breeze	Large waves form, many whitecaps, foam everywhere, more spray	4-5	1/4-2 hr.
7	28-33	Moderate gale	Sea heaps up, streaks of foam spindrift begins	5-6	1/2-3 hr.
8	34-40	Fresh gale	Moderately-high long waves, crests into spindrift, well-marked streaks of foam	6-7	1/2-3 hr.
9	41-47	Strong gale	High waves, sea rolls, dense streaks, spray affects visibility	7-9	1/2-4 hr.

 

**	Wave height is dependent on water depth and length of time that the wind has been blowing. These are typical heights for lakes, bays and estuaries. The above conditions and wave heights should prevail after the times indicated. Don't neglect the effect of large numbers of powerboats on enclosed bodies of water in estimating wave heights.
***	These attainment times are for winds of constant or increasing intensities. For decreasing intensities, surface characteristics will have to be relied upon; for example, one-foot glassy-smooth waves still indicate BN=0 wind conditions.