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A scale of wind force and speed, originally devised in 1806 by Sir Francis Beaufort, a British naval officer.

Beaufort Number   International Description   Wind Speed (mph)
0                 calm                        less than 1
1                 light air                   1-3
2                 light breeze                4-7
3                 gentle breeze               8-12
4                 moderate breeze             13-18
5                 fresh breeze                19-24
6                 strong breeze               25-31
7                 moderate gale               32-38
8                 fresh gale                  39-46
9                 strong gale                 47-54
10                whole gale                  55-63
11                storm                       64-72
12-17             hurricane                   73 and above
Beaufort Scale

The international scale used to describe wind speed.

Devised by Sir Francis Beaufort, a British admiral of the time of Nelson. It rates winds from calm to hurricane - "that which no canvas could withstand" as Beaufort originally put it.
For official readings, measurements are taken at 32.8ft (10m) above the ground where speeds will be considerably more than at ground level. When a meteorological report says that a wind is something like "Gale Force 9", it is the Beaufort scale to which it refers.

To suppliment CentrX's scale above, here is a verbose description of the numbers on the scale:

0:   Calm, smoke rises vertically.
1:   Wind direction shown by smoke, but not flags.
2:   Wind felt on face, leaves whisper, flags move.
3:   Leaves and thin twigs move, pennants extended fully.
4:   Paper and dust lifted, twigs and thin branches move.
5:   Small trees in leaf begin to sway, white horses on lakes.
6:   Thicker branches move, whistling in telephone lines, umbrellas difficult to use.
7:   Whole trees moving, resistance to movement against wind perceptible.
8:   Twigs broken from trees, movement in open difficult.
9:   Minor damage to houses (awnings and TV antennas)
10:  Trees uprooted, major damage to houses.
11:  Widespread damage.
12:  Severe destruction.

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