Antroposi — the Beaufort wind force scale | antroposi.org
All thirteen force numbers with wind speeds in knots, km/h and m/s, wave heights, and the sea and land conditions each one describes.
The Beaufort scale assigns each integer from 0 to 12 a band of wind speed, a wave height for the open ocean, and a fixed set of observable conditions at sea and on land. Force 0 is calm, below 1 knot, with the sea like a mirror and smoke rising vertically. Force 12 is hurricane-force, 64 knots and above (118 km/h and above), with waves of 14 metres and above, the air filled with foam and spray and visibility very seriously affected.
Intermediate forces fill the range in bands. Force 4, a moderate breeze, is 11–16 knots with small waves and fairly frequent white horses. Force 8, a gale, is 34–40 knots with moderately high waves and twigs breaking off trees. Force 10, a storm, is 48–55 knots with very high waves and trees uprooted.
Wind speed on the modern scale follows the empirical relation v = 0.836 × B^1.5 m/s, equivalently v = 1.625 × B^1.5 knots, where v is the wind speed at 10 metres above the sea surface and B is the Beaufort number. The formula recovers a single speed for any force number; the table gives the band that number covers.
On narrow screens, swipe or scroll the plate sideways.
Wave heights in the scale are stated for conditions in the open ocean, not along the shore. Coastal observers should read the sea-state wording rather than the metre figures, because fetch, depth and coastline shape change the waves a given wind produces.
The description columns run in parallel: sea state on one side, land effects on the other, both pointing at the same force number. Vertical smoke marks force 0, white horses force 4, breaking twigs force 8, uprooted trees force 10, foam-filled air force 12.
Use this table as the lookup: find the force number, read across for knots, km/h, m/s, wave height and conditions, and use the conversion relation on the estimating page when you need a speed between bands.
Further reading