The Beaufort Scale relates wind speed to observed conditions at sea or on land.
| Beaufort Number | Description | Wind Speed | Wave Height | Sea Conditions | Land Conditions | Weather Flag | |||
|---|---|---|---|---|---|---|---|---|---|
| km/h | mph | kt | m | ft | |||||
| 0 | Light | <1 | <1 | <1 | 0 | 0 | Flat. | Calm; smoke rises vertically. | N/A |
| 1 | Light | 1-5 | 1-3 | 1-3 | 0-0.1 | 0-0.2 | Ripples without crests. | Smoke drift indicates wind direction; wind vanes are still. | N/A |
| 2 | Light | 6-11 | 4-7 | 4-6 | 0.2 | 0.5-1 | Small wavelets; crests of glassy appearance, not breaking. | Wind felt on exposed skin; leaves rustle and wind vanes begin to move. | N/A |
| 3 | Gentle | 12-19 | 8-12 | 7-10 | 0.6 | 2-3 | Large wavelets; crests begin to break; scattered whitecaps | Leaves and small twigs constantly moving; light flags extended. | N/A |
| 4 | Moderate | 20-28 | 13-18 | 11-16 | 1 | 3.5-5 | Small waves with breaking crests; numerous whitecaps. | Dust and loose paper raised; small branches begin to move. | N/A |
| 5 | Fresh | 29-38 | 19-24 | 17-21 | 2 | 6-8 | Moderate waves; some spray. | Moderately size branches move; small trees in leaf begin to sway. | N/A |
| 6 | Strong | 39-49 | 25-31 | 22-27 | 3 | 9.5-13 | Large waves begin to form; whitecaps common; more spray. | Large branches in motion; whistling heard in overhead wires. |  |
| 7 | Strong | 50-61 | 32-38 | 28-33 | 4 | 13.5-19 | Sea heaps up; some foam is blown into streaks. | Whole trees in motion; effort needed to walk against the wind. | |
| 8 | Gale | 62-74 | 39-46 | 34-40 | 5.5 | 18-28 | Moderately high waves with breaking crests forming spindrift; foam blown in streaks. | Small branches break; progress on foot is seriously impeded. |
|
| 9 | Gale | 75-88 | 47-54 | 41-47 | 7 | 23-32 | High waves whose crests sometimes roll over; dense foam is blown along wind direction; visibility is greatly reduced. | Slight structural damage. |
|
| 10 | Whole gale | 89-102 | 55-63 | 48-55 | 9 | 29-41 | Very high waves with overhanging crests; densely blown foam; visibility further reduced. | Trees are broken off or uprooted; significant structural damage. |  |
| 11 | Whole gale | 103-117 | 64-72 | 56-63 | 11.5 | 39-46 | Extremely high waves; large patches of foam cover much of the sea; visibility severely reduced. | Widespread damage. | |
| 12 | Hurricane | >117 | >72 | >63 | >11.5 | >46 | Huge waves; sea is completely white with driving spray; visibility extremely poor. | Widespread devastation. | |