Antroposi — the Beaufort wind force scale | antroposi.org
How to assign a Beaufort number from sea state or land effects, and how the B^1.5 formula converts that number into a wind speed.
Start with what you can see. The scale's description columns are the working instrument: a mirror sea with vertical smoke is force 0, small waves with frequent white horses is force 4, twigs breaking off trees is force 8, uprooted trees is force 10, foam-filled air with visibility very seriously affected is force 12. Match the wording, take the number that carries it.
If both columns are available, prefer the sea state when at sea and the land effects when ashore. Buildings, trees and hills shelter a land observation while the open sea does not, so the two can disagree at the same wind speed; record which column you used.
Wave heights in the scale are for the open ocean, not the shore. Near a coastline, read the sea-state wording rather than the metre figures, because fetch, depth and coastline shape all change the waves a given wind produces.
On narrow screens, swipe or scroll the plate sideways.
To convert the number to a speed, use v = 0.836 × B^1.5 m/s, or v = 1.625 × B^1.5 knots, with v measured at 10 metres above the sea surface. Raise the force number to the power 1.5 and multiply: force 4 gives 0.836 × 4^1.5 ≈ 6.7 m/s, force 12 gives 0.836 × 12^1.5 ≈ 34.6 m/s, matching the 64-knot floor of the hurricane-force band.
The relation is an empirical fit, not a law of fluid dynamics. Use the published table bands when you need the official range for a force number, and the formula when you need a single point estimate or a value between bands.
Common failure to avoid: quoting open-ocean wave metres for a beach observation, or reading the land column while standing on an exposed headland where shelter does not apply.
Further reading