The Beaufort scale converts wind into thirteen forces, from 0 —calm— to 12 —hurricane force. It began as a way to describe what wind did to a ship and the sea; its levels are now associated with measured speed ranges. It remains especially useful in marine forecasts and for communicating wind intensity intuitively.
Beaufort is not a damage scale or a complete forecast. It generally classifies mean wind, while a short gust can be much faster. A given force does not always create the same waves near a coast either.
Beaufort table: knots and km/h
| Beaufort | Common description | Knots | km/h |
|---|---|---|---|
| 0 | Calm | <1 | 0–1 |
| 1 | Light air | 1–3 | 1–5 |
| 2 | Light breeze | 4–6 | 6–11 |
| 3 | Gentle breeze | 7–10 | 12–19 |
| 4 | Moderate breeze | 11–16 | 20–28 |
| 5 | Fresh breeze | 17–21 | 29–38 |
| 6 | Strong breeze | 22–27 | 39–49 |
| 7 | Near gale | 28–33 | 50–61 |
| 8 | Gale | 34–40 | 62–74 |
| 9 | Strong gale | 41–47 | 75–88 |
| 10 | Storm | 48–55 | 89–102 |
| 11 | Violent storm | 56–63 | 103–117 |
| 12 | Hurricane force | 64+ | >117 |
Limits can differ by one kilometre per hour because organisations round conversions differently. This table follows the ranges published by the Royal Netherlands Meteorological Institute, which relates Beaufort to a ten-minute mean. One knot is exactly one nautical mile per hour, approximately 1.852 km/h.
What does the scale actually observe?
The original idea was to estimate wind strength without an anemometer from visible effects. On land, lower forces are indicated by smoke, leaves and small twigs. By forces 5 and 6, larger branches move and umbrellas become difficult to use; at 7 or 8, walking or cycling into the wind becomes hard. The US National Weather Service provides a table of visual clues and possible effects.
At sea, the descriptions progress from a smooth surface to ripples, white crests, wind-blown foam and increasingly organised waves. These clues support an estimate, but an instrument provides a more precise measurement.
Why Beaufort is not the maximum gust
A Beaufort force corresponds to an average speed over a stated period. A gust is a short-lived increase above that average. A forecast can therefore show mean wind at force 5 and gusts falling within much higher speed bands. The figures do not conflict; they describe different quantities.
Our guide to mean wind and gusts explains the distinction. For small craft, cycling, work at height or temporary structures, the forecast gust may matter more than the Beaufort number.
Why the same force does not always make the same waves
Sea state depends on wind speed, but also on how long it blows, the distance over water —fetch—, depth, currents and swell arriving from elsewhere. Force 6 that has just developed close to shore need not produce the same sea as force 6 blowing over open water for many hours.
Significant wave height is a separate variable and should not be inferred automatically from Beaufort. Our guide to wave height, period and direction shows how to combine them.
How to read the most common forces
- 0–3: calm to gentle breeze; very light objects and small craft can already be affected.
- 4–5: moderate to fresh breeze; white crests and effort against the wind increase.
- 6–7: strong breeze to near gale; large branches move continuously and many activities require experience or postponement.
- 8–9: gale to strong gale; twigs can break and blown foam and spray may reduce visibility at sea.
- 10–12: storm to hurricane force; highly dangerous conditions with potential for extensive damage.
These effects are indicative. The condition of trees, buildings, soil and exposed objects changes the actual impact.
“Hurricane force” does not necessarily mean a hurricane
Beaufort 12 describes wind intensity. A hurricane or tropical cyclone is a weather system defined by its structure and development, not only by one local wind measurement. An extratropical low or convective event can also produce hurricane-force gusts without becoming a hurricane.
The Met Office explains that the Beaufort scale remains in use, particularly in marine forecasting. Always check whether a bulletin refers to sustained wind, an average, gusts or Beaufort force.
Using Beaufort in practice
- Identify the unit and averaging period used by the source.
- Separate mean wind from the predicted maximum gust.
- At the coast, also check wave height, period and direction.
- Consult official warnings; a Beaufort number does not replace one.
- Adapt your decision to the activity, experience and local exposure.
The scale turns an abstract measurement into understandable effects. Its best use is not guessing the weather from one tree, but combining observations, instruments, forecasts and warnings to make a prudent decision.