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The Beaufort Scale

The scale was created in 1805 by Sir Francis Beaufort, a British naval officer and hydrographer. At that time naval officers made regular weather observations, but there was no scale and so they could be very subjective - one man's "stiff breeze" might be another's "calm conditions". The initial scale from zero to 12 did not reference wind speed numbers, but related qualitative wind conditions to effects on the sails of a man of war, then the main ship of the Royal Navy, from "just sufficient to give steerage" to "that which no canvas [sails] could withstand."

The scale was made a standard for ship's log entries on Royal Navy vessels in the late 1830s. The scale was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. In 1906, with the advent of steam power, the descriptions were changed to how the sea, not the sails, behaved and extended to land observations. Rotations to scale numbers were standardised only in 1923. George Simpson, Director of the UK Meteorological Office, was responsible for this and for the addition of the land-based descriptors. The measure was slightly altered some decades later to improve its utility for meteorologists. Wind speed on the Beaufort scale can be expressed by the formula:
vv = 0.837 B3/2 m/s

where v is wind speed and B is Beaufort scale number.

 
No. Knots Mph Description Effects at sea Effects on land
0 0 0 Calm Sea like a mirror Smoke rises vertically
1 1-3 1-3 Light air Ripples, but no foam crests Smoke drifts in the wind
2 4-6 4-7 Light breeze Small wavelets Leaves rustle. Wind felt on face
3 7-10 8-12 Gentle breeze Large wavelets crests, not breaking Small twigs in constant motion. Light flags extended
4 11-16 13-18 Moderate wind Numerous whitecaps Dust, leaves and loose paper raised. Small branches move.
5 17-21 19-24 Fresh wind Many whitecaps, some spray Small trees sway
6 22-27 25-31 Strong wind Larger waves form. Whitecaps everywhere. More spray Large branches move. Whistling in phone wires. Difficult to use umbrellas
7 28-33 32-38 Very strong wind White foam from breaking waves begins to be blown in streaks Whole trees in motion
8 34-40 39-46 Gale Edges of wave crests begin to break into spindrift Twigs break off trees. Difficult to walk
9 41-47 47-54 Severe gale High waves. Sea begins to roll. Spray may reduce visibility Chimney pots and slates removed
10 48-55 55-63 Storm Very high waves with overhanging crests. Blowing foam gives sea a white appearance Trees uprooted. Structural damage
11 56-63 64-72 Severe storm Exceptionally high waves Widespread damage
12 63 73 Hurricane force Air filled with foam. Sea completely white. Visibility greatly reduced Widespread damage. Very rarely experienced on land