Science & Units
Beaufort Wind Scale
The Beaufort scale is unusual among measurement systems: it was built to describe what wind does, not how fast it moves. Francis Beaufort, a Royal Navy officer, assigned numbers to the sails a frigate could carry in 1805; the speed ranges attached to those numbers came much later, when anemometers made it possible to measure what sailors had been judging by eye. That history still shapes the table — the descriptive columns are the original definition, and the numbers are a retrofit.
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The practical consequence is that the scale is coarse on purpose. Each force covers a range of speeds, roughly 1.5 times the one below it, so force 6 covers 22 to 27 knots and force 7 covers 28 to 33. Two anemometers reading 26 and 29 knots are in different forces even though they are only 3 knots apart. When a forecast says "force 6" it is telling you what to expect, not giving you a number to check against your own instrument.
The speed columns here are converted rather than copied: the WMO defines each force by a knot range, and every km/h, mph and m/s figure is that range multiplied by the exact conversion factor (1 knot = 1.852 km/h = 0.514444 m/s). If you need the boundary in a unit that is not in the table, convert the knot value and you will land in exactly the same place.
Beaufort force, the wind speed that defines it, and what it looks like
| Force | Knots | km/h | mph | m/s | Sea state | On land |
|---|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 | Sea like a mirror | Smoke rises vertically |
| 1 | 1–3 | 1.9–5.6 | 1.2–3.5 | 0.5–1.5 | Small ripples, no foam crests | Smoke drifts; weather vanes still |
| 2 | 4–6 | 7.4–11.1 | 4.6–6.9 | 2.1–3.1 | Small wavelets, glassy crests | Leaves rustle; you feel wind on your face |
| 3 | 7–10 | 13–18.5 | 8.1–11.5 | 3.6–5.1 | Large wavelets, scattered whitecaps | Leaves and small twigs in constant motion |
| 4 | 11–16 | 20.4–29.6 | 12.7–18.4 | 5.7–8.2 | Small waves, frequent whitecaps | Dust and loose paper lift; small branches move |
| 5 | 17–21 | 31.5–38.9 | 19.6–24.2 | 8.7–10.8 | Moderate waves, many whitecaps, some spray | Small trees in leaf begin to sway |
| 6 | 22–27 | 40.7–50 | 25.3–31.1 | 11.3–13.9 | Larger waves, white foam crests everywhere | Large branches move; umbrellas are hard to hold |
| 7 | 28–33 | 51.9–61.1 | 32.2–38 | 14.4–17 | Sea heaps up, foam blown in streaks | Whole trees in motion; walking into it is work |
| 8 | 34–40 | 63–74.1 | 39.1–46 | 17.5–20.6 | Moderately high waves, spindrift | Twigs break off; progress on foot is slow |
| 9 | 41–47 | 75.9–87 | 47.2–54.1 | 21.1–24.2 | High waves, dense foam, spray reduces visibility | Slight structural damage: tiles, gutters, branches |
| 10 | 48–55 | 88.9–101.9 | 55.2–63.3 | 24.7–28.3 | Very high waves, sea surface white | Trees uprooted; considerable structural damage |
| 11 | 56–63 | 103.7–116.7 | 64.4–72.5 | 28.8–32.4 | Exceptionally high waves, sea covered in foam | Widespread damage to buildings and roofs |
| 12 | 64+ | 118.5+ | 73.6+ | 32.9+ | Air filled with foam and spray | Devastation; very rarely experienced inland |
The ranges leave gaps at whole knots (1–3, then 4–6) because the scale is defined on rounded knot values. The km/h, mph and m/s columns are converted from knots, so they inherit those gaps.
Turning a measured wind speed into a force
| km/h | mph | Knots | Beaufort force | What it means |
|---|---|---|---|---|
| 5 | 3.1 | 2.7 | 1 | Smoke drifts; weather vanes still |
| 10 | 6.2 | 5.4 | 2 | Leaves rustle; you feel wind on your face |
| 20 | 12.4 | 10.8 | 4 | Dust and loose paper lift; small branches move |
| 30 | 18.6 | 16.2 | 4 | Dust and loose paper lift; small branches move |
| 40 | 24.9 | 21.6 | 6 | Large branches move; umbrellas are hard to hold |
| 50 | 31.1 | 27 | 6 | Large branches move; umbrellas are hard to hold |
| 60 | 37.3 | 32.4 | 7 | Whole trees in motion; walking into it is work |
| 80 | 49.7 | 43.2 | 9 | Slight structural damage: tiles, gutters, branches |
| 100 | 62.1 | 54 | 10 | Trees uprooted; considerable structural damage |
| 120 | 74.6 | 64.8 | 12 | Devastation; very rarely experienced inland |
Read down the knots column to the first row that contains your measurement, then across. A 50 km/h wind is 27 knots, which is force 6 — the point where holding an umbrella stops working.
Where the named thresholds sit on the scale
| Threshold | Knots | km/h | Beaufort force |
|---|---|---|---|
| Light breeze begins | 4 | 7.4 | 2 |
| Strong breeze — small craft advisory | 22 | 40.7 | 6 |
| Near gale | 28 | 51.9 | 7 |
| Gale | 34 | 63 | 8 |
| Storm | 48 | 88.9 | 10 |
| Violent storm | 56 | 103.7 | 11 |
| Hurricane force | 64 | 118.5 | 12 |
Tropical cyclones are named at 34 knots (force 8) and become hurricanes at 64 knots (force 12). Those are the same two numbers that appear in shipping forecasts as gale and hurricane force.
How to Use These Tables
- Read the force from the knots column, not from the description: descriptions are subjective and depend on what is around you. Force 5 over open water looks calmer than force 5 in a city street funnelling between buildings.
- For marine forecasts, use the named thresholds table — gale is force 8 (34 knots) and hurricane force is 12 (64 knots). Small craft advisories typically start at force 6.
- To go from a measured speed to a force, divide km/h by 1.852 to get knots, then find the first row whose upper bound your value falls below. The gaps between ranges (1–3, then 4–6) are rounding artefacts and never leave a real measurement unclassified.
- For anything that depends on the actual load — sail area, crane limits, roof uplift — use the measured speed and the gust factor, not the force number. Beaufort is a description, and descriptions are not design inputs.
Common Questions
Why do the knot ranges skip numbers, like 1–3 and then 4–6?
The scale is defined on whole knots, so each force covers a band of integer values and the boundaries fall between them. A wind of 3.4 knots rounds into force 1, and 3.6 knots rounds into force 2. The gaps are a property of rounding to integers, not a hole in the definition — no real wind speed is unclassified.
Is Beaufort still used officially?
Yes, in marine and aviation weather. Shipping forecasts, port warnings and many national meteorological services still issue wind in Beaufort force, because it conveys expected impact in one digit. Scientific and engineering work uses measured speed, usually in m/s or knots.
How does force 12 relate to hurricane categories?
Force 12 starts at 64 knots, which is the same threshold as a Category 1 hurricane on the Saffir-Simpson scale. The scales then diverge: Saffir-Simpson runs to Category 5 at 137 knots, while Beaufort stops at 12 and calls anything above it force 12. So a Category 5 hurricane is force 12, but force 12 covers everything from a minimal hurricane upward.
Why is force 0 defined as less than 1 knot rather than zero?
Because calm is a range, not a point. Smoke rising vertically is the test, and that holds for any wind below about 1 knot. Defining it as exactly zero would leave a gap between dead calm and the first measurable movement.
Can I use Beaufort to estimate wind speed from the sea state alone?
Roughly, yes — that is how the scale was used for two centuries. Wave height and whitecap coverage are the usual cues, but they also depend on fetch (how far the wind has blown over water) and on how long it has been blowing. A short fetch produces smaller waves for the same wind, so the sea-state description can understate the force.
More Science & Units
Sources
- Manual on Codes (WMO-No. 306), Beaufort scale of wind force — World Meteorological Organization
- Beaufort wind scale — coast and sea guides — UK Met Office
- JetStream — wind and marine weather — US National Weather Service
- The Beaufort Scale of Wind Force, report 3 — World Meteorological Organization
Filed with Science calculations.