The Beaufort scale — from calm to hurricane in 13 steps

Admiral Francis Beaufort designed the scale in 1805 as a way for ship officers to record wind by observed effects on sail and sea. The scale is qualitative in origin but was formally tied to wind speed ranges in 1946, and it remains the standard for marine forecasts worldwide.

The scale (10-minute mean wind at 10 m)

0 Calm — 0–1 kn (0–2 km/h) > 1 Light air — 1–3 kn > 2 Light breeze — 4–6 kn > 3 Gentle breeze — 7–10 kn > 4 Moderate breeze — 11–16 kn > 5 Fresh breeze — 17–21 kn > 6 Strong breeze — 22–27 kn > 7 Near gale — 28–33 kn > 8 Gale — 34–40 kn > 9 Strong gale — 41–47 kn > 10 Storm — 48–55 kn > 11 Violent storm — 56–63 kn > 12 Hurricane — 64+ kn

When to trust it

Beaufort was designed for wind at sea, measured 10 m above the surface. Onshore anemometers, low-altitude readings, and gusty terrain all give different numbers. The values above are 10-minute means — a gust in a 'strong breeze' warning can easily hit gale force momentarily.

The formula

For quick calculation, wind speed in knots ≈ (Beaufort force)^1.5 × 1.87. Force 6: 6^1.5 × 1.87 = 27.5 kn. Force 10: 10^1.5 × 1.87 = 59.1 kn. Close enough to the table for mental math.

Worked example — sail reduction

A cruising yacht typically reefs the mainsail at Beaufort 5 (17–21 kn = 31–39 km/h). At Beaufort 7, most cruising boats have gone to storm sails. At Beaufort 10 (48–55 kn = 89–102 km/h), the boat is fighting for survival regardless of sail plan.

The five-second recap

Force 4 = 11–16 kn (moderate breeze, sailboats happy). Force 8 = gale (~34–40 kn, freighters slow). Force 12 = 64+ kn (hurricane). Beaufort^1.5 × 1.87 gives kn.