Antroposi — the Beaufort wind force scale | antroposi.org
independent recordantroposi.orgindependent reference record
Antroposi — the Beaufort wind force scale | antroposi.org

Force 0 to 12, explained

The Beaufort scale is an empirical measure linking wind speed to observed conditions at sea or on land. It was devised in 1805 and has been in continuous standard use for over two centuries.

Modern wind speed on the scale follows the empirical relation v = 0.836 × B^1.5 m/s, where B is the force number and v is measured 10 metres above the sea surface.

Force numbers run 0 to 12; wave heights quoted in the scale refer to the open ocean, not to conditions along the shore.

What does each Beaufort number mean in knots, km/h and wave height?

Each force number from 0 to 12 maps to a wind speed band in knots, a wave height range and a fixed set of observable sea and land conditions.

Force 0 is calm: below 1 knot, the sea like a mirror, smoke rising vertically. Force 4, a moderate breeze, runs 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.

Force 12, hurricane-force, begins at 64 knots (118 km/h and above) with waves of 14 metres and above, the air filled with foam and spray, and visibility very seriously affected. Wave heights in the scale are stated for the open ocean, not along the shore, so coastal observers should expect different numbers for the same force.

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.

  • Force 0 — calm, below 1 knot, mirror sea, smoke rises vertically
  • Force 4 — moderate breeze, 11–16 knots, small waves, frequent white horses
  • Force 8 — gale, 34–40 knots, moderately high waves, twigs break off trees
  • Force 10 — storm, 48–55 knots, very high waves, trees uprooted
  • Force 12 — hurricane-force, 64 knots and above, waves 14 m and above

How do you convert a Beaufort number to a wind speed?

Raise the force number to the power 1.5 and multiply by 0.836 for m/s, or by 1.625 for knots.

The relation v = 0.836 × B^1.5 m/s is the documented empirical link between the force number and wind speed at 10 metres above the sea surface. The same relation in knots is v = 1.625 × B^1.5. Because the exponent is 1.5, speed grows faster than the force number: doubling the force multiplies speed by about 2.8.

Worked check: force 4 gives 0.836 × 4^1.5 = 0.836 × 8 = 6.69 m/s, which rounds into the published 11–16 knot band once gusts and averaging are accounted for. Force 12 gives 0.836 × 12^1.5 ≈ 34.6 m/s, matching the 64-knot floor of the hurricane-force band.

The formula is an empirical fit, not a law of fluid dynamics; use the published table bands when you need the official range, and the formula when you need a single point estimate between bands.

  • Identify the Beaufort number B from 0 to 12
  • Compute B^1.5 (B raised to the power 1.5)
  • Multiply by 0.836 for metres per second, or by 1.625 for knots
  • Multiply m/s by 1.944 to get knots, or knots by 0.5144 to get m/s
  • Compare the result with the published band for that force number
The original scale never mentioned wind speed: it described what a frigate's sails did, from just sufficient to give steerage to that which no canvas sails could withstand.

The scale in five dated steps

How do you read a force number off what you see?

Match the state of the sea or the effect on land to the description column, then take the force number that carries that description.

The description columns are the working end of the scale. 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. Land effects and sea state are listed side by side so either can be used.

If both columns are available, prefer the sea state when at sea and the land effects when ashore. If the two disagree, the scale is being read across different exposure: buildings, trees and hills shelter the land observation while the open sea does not.

Wave heights quoted in the scale are for the open ocean. Along a shore, the same force produces different wave heights, so a coastal observer should use the sea-state wording rather than the metre figures.

Where did the scale come from?

Francis Beaufort devised it in 1805; it became a Royal Navy standard in the late 1830s, was accepted internationally in 1853, rewritten for sea state in 1916 and tied to anemometers in 1923.

The scale was devised in 1805 by Francis Beaufort, a hydrographer and Royal Navy officer, while serving on HMS Woolwich. The initial scale of 13 classes, zero to 12, did not reference wind speed numbers; it related qualitative wind conditions to effects on the sails of a frigate, from 'just sufficient to give steerage' to 'that which no canvas sails could withstand'.

It was made a standard for ship's log entries on Royal Navy vessels in the late 1830s, and in 1853 it was accepted as generally applicable at the First International Meteorological Conference in Brussels. In 1916 the descriptions were changed to how the sea, not the sails, behaved, and extended to land observations. Anemometer rotations to scale numbers were standardised in 1923 under George Simpson of the UK Meteorological Office.

  • 1805 — devised by Francis Beaufort on HMS Woolwich
  • Late 1830s — standard for Royal Navy ship's log entries
  • 1853 — accepted at the First International Meteorological Conference, Brussels
  • 1916 — descriptions rewritten from sails to sea state, extended to land
  • 1923 — anemometer rotations standardised under George Simpson

Why do wave heights differ near the shore?

The scale states wave heights for the open ocean, so shoreline observations of the same force will show different waves.

Wave heights in the scale are stated for conditions in the open ocean, not along the shore. Fetch, depth and coastline shape all change the waves that a given wind produces, so a force 8 offshore can arrive at a beach as a much smaller or much larger breaker depending on the seabed.

For consistent reporting, the sea-state wording of each force number is the portable part of the scale: white horses, moderately high waves, very high waves, and the foam-and-spray description at force 12 travel across coastlines better than the metre figures do.

Observers on land can still assign a force number from the land column — smoke behaviour, twig breakage, tree uprooting — without needing any wave measurement at all.

  • Open ocean: use the metre figures as published
  • Shoreline: use the sea-state wording, not the metre figures
  • Land: use the land-effect column, from vertical smoke to uprooted trees

Key values

Selected Beaufort forces: wind speed and the conditions each one describes
ForceWind speed (knots)Wind speed (km/h)Wave heightSea conditionLand condition
0below 1below 2mirrorSea like a mirrorSmoke rises vertically
411–1620–30small wavesFairly frequent white horsesDust and loose paper raised
834–4063–74moderately high wavesModerately high wavesTwigs breaking off trees
1048–5589–102very high wavesVery high wavesTrees uprooted
1264 and above118 and above14 m and aboveAir filled with foam and sprayWidespread damage
Selected Beaufort forces: wind speed and the conditions each one describes

Reading the sea and the land, force by force

Each card pairs a force number with its wind band and its observable conditions.

1805Francis Beaufort devises the scale on HMS Woolwich, with 13 classes tied to frigate sailresponse.Late 1830sThe scale becomes standard for ship's log entries on Royal Navy vessels.1853Accepted as generally applicable at the First International Meteorological Conference inBrussels.1916Descriptions rewritten from how sails behaved to how the sea behaved, extended to landobservations.1923Anemometer rotations standardised to scale numbers under George Simpson of the UK MeteorologicalOffice.
From frigate sails to anemometers, 1805–1923
Beaufort numberEmpirical relationWhite horsesGale
The key terms of this guide, drawn to one scale

Sources

Reference material on the Beaufort scale, its 1805 origin and its 1916 and 1923 revisions.