Beaufort wind scale
Devised in 1805 by Rear-Admiral Sir Francis Beaufort of the Royal Navy, the Beaufort scale describes wind force by its observable effect on the sea (and, ashore, on smoke, vegetation, and structures) rather than by an instrument reading. It became the standard wind report on Royal Navy ships in 1838 and remains the canonical short-form wind descriptor in marine weather bulletins worldwide.
Wind speeds in the table are sustained 10-minute means at 10 m above sea level (the WMO standard). Gusts can exceed the mean by 30–50%. The scale ends at force 12 in its original form; some national services extend it to force 17 for tropical cyclones.
Scale
| Force | Term | Knots | m/s | km/h | mph |
|---|---|---|---|---|---|
| 0 | Calm | < 1 | 0–0.2 | < 1 | < 1 |
| 1 | Light air | 1–3 | 0.3–1.5 | 1–5 | 1–3 |
| 2 | Light breeze | 4–6 | 1.6–3.3 | 6–11 | 4–7 |
| 3 | Gentle breeze | 7–10 | 3.4–5.4 | 12–19 | 8–12 |
| 4 | Moderate breeze | 11–16 | 5.5–7.9 | 20–28 | 13–18 |
| 5 | Fresh breeze | 17–21 | 8.0–10.7 | 29–38 | 19–24 |
| 6 | Strong breeze | 22–27 | 10.8–13.8 | 39–49 | 25–31 |
| 7 | Near gale | 28–33 | 13.9–17.1 | 50–61 | 32–38 |
| 8 | Gale | 34–40 | 17.2–20.7 | 62–74 | 39–46 |
| 9 | Strong gale | 41–47 | 20.8–24.4 | 75–88 | 47–54 |
| 10 | Storm | 48–55 | 24.5–28.4 | 89–102 | 55–63 |
| 11 | Violent storm | 56–63 | 28.5–32.6 | 103–117 | 64–72 |
| 12 | Hurricane | ≥ 64 | ≥ 32.7 | ≥ 118 | ≥ 73 |
Sea and land criteria
- 0Calm< 1 kt
Sea: Sea like a mirror.
Land: Smoke rises vertically.
- 1Light air1–3 kt
Sea: Ripples with the appearance of scales, no foam crests.
Land: Smoke drifts; wind vanes do not move.
- 2Light breeze4–6 kt
Sea: Small wavelets, glassy crests, not breaking.
Land: Wind felt on face; leaves rustle; vanes move.
- 3Gentle breeze7–10 kt
Sea: Large wavelets, crests begin to break, scattered whitecaps.
Land: Leaves and small twigs in constant motion; light flag extended.
- 4Moderate breeze11–16 kt
Sea: Small waves becoming longer; fairly frequent whitecaps.
Land: Dust raised; small branches move.
- 5Fresh breeze17–21 kt
Sea: Moderate waves of pronounced long form; many whitecaps; some spray.
Land: Small trees in leaf begin to sway.
- 6Strong breeze22–27 kt
Sea: Large waves begin to form; foam crests extensive; some airborne spray.
Land: Large branches in motion; whistling in overhead wires; umbrella use difficult.
- 7Near gale28–33 kt
Sea: Sea heaps up; white foam from breaking waves blown in streaks along the wind.
Land: Whole trees in motion; inconvenience felt walking against wind.
- 8Gale34–40 kt
Sea: Moderately high waves of greater length; edges of crests break into spindrift.
Land: Twigs break from trees; progress against the wind generally impeded.
- 9Strong gale41–47 kt
Sea: High waves; dense streaks of foam; crests begin to roll over; spray reduces visibility.
Land: Slight structural damage (chimney pots and slates removed).
- 10Storm48–55 kt
Sea: Very high waves with long overhanging crests; sea surface takes on a white appearance.
Land: Trees uprooted; considerable structural damage. Seldom inland.
- 11Violent storm56–63 kt
Sea: Exceptionally high waves; sea completely covered with long white patches of foam.
Land: Very rarely experienced; widespread damage.
- 12Hurricane≥ 64 kt
Sea: Air filled with foam and spray; sea completely white with driving spray; visibility seriously affected.
Land: Devastation.
Gale, storm, and hurricane warnings
Marine forecasts use named warnings tied to Beaufort thresholds:
- · Gale warning — winds of force 8 (34–40 kt) imminent or expected.
- · Storm warning — winds of force 10 (48–55 kt).
- · Hurricane-force warning — winds of force 12 (≥ 64 kt), used in non-tropical contexts.
- · Tropical cyclone warning — separate Saffir–Simpson / regional category scales for named systems.
See also
- · Douglas sea state scale — wave heights generated by these wind forces.
- · Unit converters — knots, m/s, km/h, mph.
- · Distress frequencies — when conditions deteriorate.