The Beaufort wind scale for lake boaters

The Beaufort scale describes wind by its effect on the water rather than by a number on an instrument. It was designed for the open sea, so waves on a lake will be smaller for the same force, but the surface signs are a good way to check a forecast against what you see.

Force	Name	Knots	On a lake
0	Calm	under 1	Glassy surface
1	Light air	1 to 3	Scaly ripples, no crests
2	Light breeze	4 to 6	Small wavelets, sails just fill
3	Gentle breeze	7 to 10	Large wavelets, a few crests break
4	Moderate breeze	11 to 16	Small waves, fairly frequent whitecaps
5	Fresh breeze	17 to 21	Moderate waves, many whitecaps, some spray
6	Strong breeze	22 to 27	Larger waves, whitecaps everywhere
7	Near gale	28 to 33	Heaped-up water, foam blown in streaks
8	Gale	34 to 40	Spray and streaks of foam across the lake

Using it on the water

Forces 2 to 4 are the sweet spot for most recreational sailing. Paddlers are most comfortable at force 2 or below. By force 5 most small open boats and paddlecraft should be off the water, and by force 6 the National Weather Service would typically have an advisory out on large lakes.

If whitecaps are already showing at the launch, the open part of the lake will be rougher than it looks from shore.