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Beaufort Scale

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The Beaufort scale is a vital tool in sailing.
by Bill Giles O.B.E.

It has been estimated that getting an accurate reading for wind direction and speed is 60% towards getting a forecast correct.

Key Points
  • The Beaufort Scale was published in 1805.
  • It has 13 steps and gives the wind speed in knots and miles per hour.
  • The scale was originally published for use at sea and was later adapted for use on land.
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Winds of the World

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But even if you don't have expensive equipment, you can get a fair idea just by looking at objects around you, and it's largely thanks to the work done by Admiral Francis Beaufort.

Measuring the exact wind speed without sophisticated equipment is not easy, even today. But it was much more difficult some 200 years ago when the large sailing ships of the Royal Navy were much more at the mercy of the weather, especially strong winds.

In 1805 Commander, later Admiral, Sir Francis Beaufort published a method of measuring the wind at sea based on what sails a frigate could safely hoist. The Beaufort Scale, as it came to be known, was adopted by the Royal Navy in 1838 when it became mandatory for all ship's log entries.

The scale had 13 steps; from force 0, where the wind was calm, to force 12, where the steady wind would be at least 64 knots or 75 miles per hour. With this scale also came descriptions of the state of the sea. From this standard, sailors were able to predict how ships would react in certain wind speeds.

It was quite a lot later in the century that the Beaufort Scale was adapted for use on the land. In many respects it, and the descriptions that go with it, are still used today.

A further set of Beaufort numbers from force 13 to force 17 was added by the United States Weather Bureau in 1955 to cope with the exceptional winds that they can get during a hurricane.

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

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