The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort wind force scale.
In the early 19th century, naval officers made regular weather observations, but there was no standard scale and so they could be very subjective – one man's "stiff breeze" might be another's "soft breeze". Beaufort succeeded in standardising the scale.
Sir Francis Beaufort
The scale was devised in 1805 by the Irish hydrographer Francis Beaufort (later Rear Admiral Sir Francis Beaufort), a Royal Navy officer, while serving on HMS Woolwich. The scale that carries Beaufort's name had a long and complex evolution from the previous work of others (including Daniel Defoe the century before) to when Beaufort was Hydrographer of the Navy in the 1830s when it was adopted officially and first used during the voyage of HMS Beagle under Captain Robert FitzRoy, later to set up the first Meteorological Office (Met Office) in Britain giving regular weather forecasts.
Below is a table showing the Beaufort Scale with speeds in knots, miles per hour and kilometers per hour. Please note that these are mean speeds, usually averaged over 10 minutes by convention, and do not capture the speed of wind gusts.
The quoted wind speed is that measured at 10 m above ground, not at the surface (which, at 2 m, may be only 50-70% of these figures)
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Calm |
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Force 0 |
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km/h <1 |
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mph <1 |
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knots <1 |
Appearance of Wind Effects
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On the sea Sea surface smooth and mirror-like |
|
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On land Calm, smoke rises vertically |
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Light air |
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Force 1 |
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km/h 1-5 |
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mph 1-3 |
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knots1-3 |
Appearance of Wind Effects
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On the sea Scaly ripples, no foam crests |
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On land Smoke drift indicates wind direction, still wind vanes |
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Light breeze |
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Force 2 |
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km/h 6-11 |
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mph 4-7 |
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knots 4-6 |
Appearance of Wind Effects
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On the sea Small wavelets, crests glassy, no breaking |
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On land Wind felt on face, leaves rustle, vanes begin to move |
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Gentle breeze |
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Force 3 |
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km/h 12-19 |
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mph 8-12 |
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knots 7-10 |
Appearance of Wind Effects
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On the sea Large wavelets, crests begin to break, scattered whitecaps |
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On land Leaves and small twigs constantly moving, light flags extended |
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Moderate breeze |
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Force 4 |
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km/h 20-18 |
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mph 13-18 |
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knots 11-16 |
Appearance of Wind Effects
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On the sea Small waves 1-4 ft. becoming longer, numerous whitecaps |
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On land Dust, leaves, and loose paper lifted, small tree branches move |
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Fresh breeze |
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Force 5 |
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km/h 29-38 |
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mph 19-24 |
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knots 17-21 |
Appearance of Wind Effects
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On the sea Moderate waves 4-8 ft taking longer form, many whitecaps, some spray |
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On land Small trees in leaf begin to sway |
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Strong breeze |
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Force 6 |
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km/h 39-49 |
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mph 25-31 |
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knots 22-27 |
Appearance of Wind Effects
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On the sea Larger waves 8-13 ft, whitecaps common, more spray |
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On land Larger tree branches moving, whistling in wires |
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Near gale |
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Force 7 |
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km/h 50-61 |
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mph 32-38 |
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knots 28-33 |
Appearance of Wind Effects
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On the sea Sea heaps up, waves 13-19 ft, white foam streaks off breakers |
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On land Whole trees moving, resistance felt walking against wind |
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Gale |
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Force 8 |
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km/h 62-74 |
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mph 39-46 |
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knots 34-40 |
Appearance of Wind Effects
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On the sea Moderately high (18-25 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks |
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On land Twigs breaking off trees, generally impedes progress |
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Strong gale |
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Force 9 |
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km/h 75-88 |
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mph 47-54 |
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knots 41-47 |
Appearance of Wind Effects
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On the sea High waves (23-32 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility |
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On land Slight structural damage occurs, slate blows off roofs |
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Storm |
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Force 10 |
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km/h 89-102 |
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mph 55-63 |
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knots 48-55 |
Appearance of Wind Effects
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On the sea Very high waves (29-41 ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility |
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On land Seldom experienced on land, trees broken or uprooted, "considerable structural damage" |
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Violent storm |
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Force 11 |
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km/h 103-117 |
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mph 64-72 |
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knots 56-63 |
Appearance of Wind Effects
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On the sea Exceptionally high (37-52 ft) waves, foam patches cover sea, visibility more reduced |
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On land Rarely expierence on land, otherwise will cause severe damage |
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Hurricane |
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Force 12 |
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km/h 118-133 |
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mph 73-82 |
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knots 64-71 |
Appearance of Wind Effects
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On the sea Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced |
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On land Never seen on land, otherwise will cause large scale of casualty |
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