Beaufort scale
Wind Beaufort wind force scale Bft
The Beaufort scale describes wind strength in 13 forces, from 0 for calm to 12 for hurricane force. Each force corresponds to a range of wind speeds.
Putting a number on a windy day
The scale was developed for sailors to describe wind by its effects, even without an instrument. Each force now has a defined speed range. Beaufort is a scale, not a speed unit: force 6 is a category, while 45 km/h is a wind speed.
The complete Beaufort scale
The table uses the customary ranges rounded to whole kilometres per hour. Effects on land are guides, not promises: tree species, foliage and ground conditions all matter.
On small screens, scroll sideways to see all columns.
| Force | Description | Wind in km/h | Possible observation |
|---|---|---|---|
| 0 | Calm |
below 1 |
Smoke rises almost vertically. |
| 1 | Light air |
1–5 |
Smoke drifts sideways. |
| 2 | Light breeze |
6–11 |
Leaves rustle; wind can be felt on the face. |
| 3 | Gentle breeze |
12–19 |
Thin twigs move continuously. |
| 4 | Moderate breeze |
20–28 |
Loose leaves are lifted. |
| 5 | Fresh breeze |
29–38 |
Small leafy trees sway. |
| 6 | Strong breeze |
39–49 |
Large branches move. |
| 7 | Near gale |
50–61 |
Whole trees sway; walking into wind takes effort. |
| 8 | Gale |
62–74 |
Twigs may break; walking becomes difficult. |
| 9 | Strong gale |
75–88 |
Roofs may be damaged. |
| 10 | Storm |
89–102 |
Trees may snap or be uprooted. |
| 11 | Violent storm |
103–117 |
Severe damage is possible. |
| 12 | Hurricane force |
118 or more |
Devastating damage is possible. |
Mean wind and gusts are different
A Beaufort forecast usually describes the mean wind, often averaged over ten minutes at a height of ten metres. Gusts are short peaks. A forecast of force 5 with gusts of 75 km/h is therefore consistent: the mean is 29–38 km/h, but individual gusts reach force 9.
What the scale cannot tell you
A force number cannot predict which branch will break or whether a particular route is safe. Duration and gustiness matter too. “Hurricane force” describes the wind speed here; it does not mean that a tropical hurricane is present.