The Beaufort scale describes wind strength from force 0 (calm) to force 12 (hurricane) by its effect on the sea. Sailors usually measure wind in knots: one knot is one nautical mile per hour, a little under 2 kilometres an hour.
Wind speed at sea is measured in knots, nautical miles per hour. A nautical mile is 1852 metres, so one knot is about 1.85 kilometres an hour, or 1.15 miles an hour. Marine forecasts, boat instruments and sailors all use knots.
The Beaufort scale was devised in the early 1800s, when it described the wind by how much sail a warship could carry. Today it describes the wind by what it does to the sea, so it can be judged by eye. The forces that matter most to sailors are:
- Force 0 to 1 (0 to 3 knots): calm or light air. Mirror-flat water or ripples.
- Force 2 (4 to 6 knots): light breeze. Small wavelets.
- Force 3 (7 to 10 knots): gentle breeze. Large wavelets, a few crests breaking.
- Force 4 (11 to 16 knots): moderate breeze. Small waves, frequent white horses.
- Force 5 (17 to 21 knots): fresh breeze. Moderate waves, many white horses.
- Force 6 (22 to 27 knots): strong breeze. Large waves, white foam crests, spray.
- Force 7 (28 to 33 knots): near gale. The sea heaps up and foam is blown in streaks.
- Force 8 (34 to 40 knots): gale. High waves, crests blowing off as spindrift.
The scale goes on to force 12, hurricane force, at 64 knots and above.
Being able to judge the force by eye is a useful skill: if the water is covered in white horses, the wind is at least 17 knots, whatever the forecast said. Remember that the force of the wind on the sails rises with the square of the wind speed, so 20 knots pushes about four times as hard as 10 knots. A small rise in the number makes a big difference on board.
More in the book: Chapter 10, When the wind gets up