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THE BEAUFORT WIND SCALE

One of the first scales to estimate wind speeds and the effects, was created by Britain's Admiral Sir Francis Beaufort (1774-1857).
He developed the scale in 1805 to help sailors estimate the winds via visual observations.
The scale starts with 0 and goes to a force of 12.
The Beaufort scale is still used today to estimate wind strengths.

The Beaufort scale
For use at sea
For use on land

BEAUFORT SCALE: Specifications and equivalent speeds for use at Sea

Beaufort number Description Wind Speed Wave height Sea conditions
mph kts m ft
0 Calm < 1 < 1 0 0 Flat.
1 Light Air 1 3 1 3 0 0.2 0 1 Ripples without crests.
2 Light Breeze 4 7 4 6 0.2 0.5 1 2 Small wavelets. Crests of glassy appearance, not breaking
3 Gentle Breeze 8 12 7 10 0.5 1 2 3.5 Large wavelets. Crests begin to break; scattered whitecaps
4 Moderate Breeze 13 18 11 16 1 2 3.5 6 Small waves with breaking crests. Fairly frequent white horses.
5 Fresh Breeze 19 24 17 21 2 3 6 9 Moderate waves of some length. Many white horses. Small amounts of spray.
6 Strong Breeze 25 31 22 27 3 4 9 13 Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.
7 Near Gale 32 38 28 33 4 5.5 13 19 Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.
8 Gale 39 46 34 40 5.5 7.5 18 25 Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray.
9 Severe Gale 47 54 41 47 7 10 23 32 High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.
10 Storm 55 63 48 55 9 12.5 29 41 Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility.
11 Violent Storm 64 72 56 63 11.5 16 37 52 Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility.
12 Hurricane = 73 = 64 = 14 = 46 Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.
 

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BEAUFORT SCALE: Specifications and equivalent speeds for use on land

FORCE  EQUIVALENT SPEED   DESCRIPTION      SPECIFICATIONS FOR USE ON LAND
     10 m above ground
     miles/hour  knots
0       0-1      0-1      Calm             Calm; smoke rises verticall.

1       1-3      1-3      Light air        Direction of wind shown by smoke drift, but not
                                           by wind vanes.
                                          
2       4-7      4-6      Light Breeze     Wind felt on face; leaves rustle; ordinary vanes
                                           moved by wind.

3       8-12     7-10     Gentle Breeze    Leaves and small twigs in constant motion; wind
                                           extends light flag.

4      13-18    11-16     Moderate Breeze  Raises dust and loose paper; small branches are
                                           moved.

5      19-24    17-21     Fresh Breeze     Small trees in leaf begin to sway; crested wavelets
                                           form on inland waters.

6      25-31    22-27     Strong Breeze    Large branches in motion; whistling heard in
                                           telegraph wires; umbrellas used with difficulty.

7      32-38    28-33     Near Gale        Whole trees in motion; inconvenience felt when
                                           walking against the wind.

8      39-46    34-40     Gale             Breaks twigs off trees; generally impedes progress.

9      47-54    41-47     Severe Gale      Slight structural damage occurs (chimney-pots and
                                           slates removed).

10     55-63    48-55     Storm            Seldom experienced inland; trees uprooted;
                                           considerable structural damage occurs.

11    64-72     56-63     Violent Storm    Very rarely experienced; accompanied by wide-spread
                                           damage.

12    73-83     64-71     Hurricane                        --


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Extracted from the Observers Handbook, Met Office