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
Beaufort number	Description	mph	kts	m	ft	Sea conditions
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