Reckoning / Weather / Beaufort scale
Weather · WMO
Beaufort scale
Francis Beaufort’s 1805 scale judged wind by its effect on a ship’s sails. Today it is read from the sea itself, and each force has a defined range of wind speed. Move the slider: the sea below is drawn to scale beside a 10-metre boat.
Force
Illustration: wave height is the published probable height for the force, wavelength assumes a steepness of about 1/10, and each wave moves at the deep-water speed for its length. Real seas also depend on fetch and duration.
The scale
| Force | Name | Knots | m/s | Probable wave (max), m | Sea | On land |
|---|---|---|---|---|---|---|
| 0 | Calm | < 1 | 0.0–0.2 | — | Sea like a mirror. | Smoke rises straight up. |
| 1 | Light air | 1–3 | 0.3–1.5 | 0.1 (0.1) | Ripples like scales, no foam crests. | Smoke drifts; vanes do not move. |
| 2 | Light breeze | 4–6 | 1.6–3.3 | 0.2 (0.3) | Small wavelets with glassy crests that do not break. | Wind felt on the face; leaves rustle. |
| 3 | Gentle breeze | 7–10 | 3.4–5.4 | 0.6 (1) | Large wavelets; crests begin to break; scattered white horses. | Leaves and twigs in constant motion; light flags extended. |
| 4 | Moderate breeze | 11–16 | 5.5–7.9 | 1 (1.5) | Small waves becoming longer; fairly frequent white horses. | Dust and loose paper lifted; small branches move. |
| 5 | Fresh breeze | 17–21 | 8.0–10.7 | 2 (2.5) | Moderate, longer waves; many white horses; some spray. | Small trees in leaf begin to sway. |
| 6 | Strong breeze | 22–27 | 10.8–13.8 | 3 (4) | Large waves form; white foam crests everywhere; probably some spray. | Large branches move; umbrellas hard to use. |
| 7 | Near gale formerly Moderate gale | 28–33 | 13.9–17.1 | 4 (5.5) | Sea heaps up; white foam from breaking waves blown in streaks. | Whole trees in motion; hard to walk against the wind. |
| 8 | Gale formerly Fresh gale | 34–40 | 17.2–20.7 | 5.5 (7.5) | Moderately high, longer waves; crests break into spindrift; well-marked foam streaks. | Twigs break off trees. |
| 9 | Strong gale | 41–47 | 20.8–24.4 | 7 (10) | High waves; dense foam streaks; crests topple and roll over; spray may reduce visibility. | Slight structural damage; slates blown off. |
| 10 | Storm formerly Whole gale | 48–55 | 24.5–28.4 | 9 (12.5) | Very high waves with long overhanging crests; sea surface white; heavy tumbling; visibility affected. | Trees uprooted; considerable structural damage. |
| 11 | Violent storm formerly Storm | 56–63 | 28.5–32.6 | 11.5 (16) | Exceptionally high waves that can hide small and medium ships; sea covered in white foam. | Widespread damage (rare inland). |
| 12 | Hurricane | 64+ | 32.7+ | 14 | Air filled with foam and spray; sea completely white; visibility very seriously affected. | Devastation. |
Sources: Beaufort wind force scale, with probable wave heights; speed limits as in WMO practice. The descriptions are written for this site.
The speed ranges follow v ≈ 0.836 B3/2 m/s, where B is the force: force 6 gives 12.3 m/s, inside its 10.8–13.8 range. The build checks that every force’s mid-speed falls inside its own range. It also found that the knot and m/s columns are two separately rounded tables: at forces 4 to 6 their boundaries differ by up to half a metre per second (16.5 knots is 8.5 m/s, but the m/s table changes force at 8.0).
Older tables named forces 7 to 10 moderate gale, fresh gale, strong gale and whole gale. Speeds were attached to the numbers only in the twentieth century; Beaufort’s own scale described the canvas a man-of-war could carry.