The Beaufort Wind Force Scale
Wind measured not in speed, but in what a ship could carry.
Origin
Devised by Commander Francis Beaufort of the Royal Navy in 1805, and made mandatory for Royal Navy log entries in 1838.
The catch
The original scale never mentioned wind speed at all. It described what sail a fully-rigged man-of-war could safely carry, which meant the same wind scored differently on a different ship.
Sea like a mirror. Smoke rises vertically.
Ripples without crests. Smoke drifts.
Small wavelets. Wind felt on the face.
Large wavelets, scattered white horses.
Small waves, frequent white horses. Dust and loose paper raised.
Moderate waves, many white horses. Small trees sway.
Large waves, foam crests. Umbrellas used with difficulty.
Sea heaps up, foam blown in streaks. Walking into the wind is inconvenient.
Moderately high waves. Twigs break off trees.
High waves, dense foam, spray affects visibility. Slates blown from roofs.
Very high waves with overhanging crests. Trees uprooted.
Exceptionally high waves. Widespread damage.
Air filled with foam and spray. Sea completely white. Beaufort’s own definition: that which no canvas could withstand.
Beaufort did not invent the idea of grading wind, but he standardised it in a form that could be applied consistently by any officer of the watch. Wind speeds were first attached to the scale in 1906, nearly a century after it was devised, and only standardised internationally in 1946. It remains in daily use in the Shipping Forecast, broadcast almost without interruption since the 1920s.