Beaufort scale

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The Beaufort scale is an empirical measure for describing wind intensity based mainly on observed sea conditions. Its full name is the Beaufort wind force scale.

The scale was created in 1806 by Sir Francis Beaufort, a British naval officer and hydrographer. The initial scale from zero to 12 did not reference wind speed numbers, but related qualitative wind conditions to effects on a naval vessel, from "just sufficient to give steerage" to "that which no canvas [sails] could withstand." The scale was made a standard for ship's log entries on Royal Navy vessels in the late 1830s.

The scale was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. Rotations to scale numbers were standardised only in 1923. George Simpson, Director of the UK Meteorological Office, was responsible for this and for the addition of the land based descriptorsTemplate:Fact. The measure was slightly altered some decades later to improve its utility for meteorologists. Wind speed on the Beaufort scale can be expressed by the formula:

v = 0.837 B^3/2 m/s

where v is wind speed and B is Beaufort scale number.

Today, hurricanes are sometimes described as Beaufort scale 12 through 16, with the standard Saffir-Simpson Hurricane Scale Category 1 equivalent to Beaufort 12, Category 2 to Beaufort 13, and so on. Category 1 tornadoes on the Fujita and TORRO scales also begin at the end of level 12 of the Beaufort scale.

Note that wave heights in the scale are for conditions in the open ocean, not along shore.

Beaufort number	Wind speed				Mean wind speed (kt / km/h / mph)	Description	Wave height		Sea conditions	Land conditions
Beaufort number	kt	km/h	mph	m/s	Mean wind speed (kt / km/h / mph)	Description	m	ft	Sea conditions	Land conditions
0	0	0	0	0-0.2	0 / 0 / 0	Calm	0	0	Flat.	Calm. Smoke rises vertically.
1	1-3	1-6	1-3	0.3-1.5	2 / 4 / 2	Light air	0.1	0.33	Ripples without crests.	Wind motion visible in smoke.
2	4-6	7-11	4-7	1.6-3.3	5 / 9 / 6	Light breeze	0.2	0.66	Small wavelets. Crests of glassy appearance, not breaking	Wind felt on exposed skin. Leaves rustle.
3	7-10	12-19	8-12	3.4-5.4	9 / 17 / 11	Gentle breeze	0.6	2	Large wavelets. Crests begin to break; scattered whitecaps	Leaves and smaller twigs in constant motion.
4	11-16	20-29	13-18	5.5-7.9	13 / 24 / 15	Moderate breeze	1	3.3	Small waves.	Dust and loose paper raised. Small branches begin to move.
5	17-21	30-39	19-24	8.0-10.7	19 / 35 / 22	Fresh breeze	2	6.6	Moderate (1.2 m) longer waves. Some foam and spray.	Smaller trees sway.
6	22-27	40-50	25-31	10.8-13.8	24 / 44 / 27	Strong breeze	3	9.9	Large waves with foam crests and some spray.	Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult.
7	28-33	51-62	32-38	13.9-17.1	30 / 56 / 35	Near gale	4	13.1	Sea heaps up and foam begins to streak.	Whole trees in motion. Effort to walk against the wind.
8	34-40	63-75	39-46	17.2-20.7	37 / 68 / 42	Gale	5.5	18	Moderately high waves with breaking crests forming spindrift. Streaks of foam.	Twigs broken from trees. Cars veer on road.
9	41-47	76-87	47-54	20.8-24.4	44 / 81 / 50	Strong gale	7	23	High waves (2.75 m) with dense foam. Wave crests start to roll over. Considerable spray.	Light structure damage.
10	48-55	88-102	55-63	24.5-28.4	52 / 96 / 60	Storm	9	29.5	Very high waves. The sea surface is white and there is considerable tumbling. Visibility is reduced.	Trees uprooted. Considerable structural damage.
11	56-63	103-117	64-72	28.5-32.6	60 / 111 / 69	Violent storm	11.5	37.7	Exceptionally high waves.	Widespread structural damage.
12	>63	>117	>72	>32.7	N/A	Hurricane	14+	46+	Huge waves. Air filled with foam and spray. Sea completely white with driving spray. Visibility very greatly reduced.	Massive and widespread damage to structures.

The scale is used in, and may be most recognizable to some from, the Shipping Forecasts broadcast on BBC Radio 4 in the United Kingdom.

This scale is also widely used in China. Taiwan uses the Beaufort scale extended in 1944 with Forces 13-17 to better represent the wind caused by typhoons. Hong Kong and mainland China keep using Force 12 as the maximum. Macau adopted a set of simpler descriptions (calm, light, gentle, moderate, strong, gale, storm, hurricane) instead of the force numbers and descriptions in the table.

In the United States, winds of Beaufort 6 or 7 result in the issuance of a small craft advisory, with force 8 or 9 winds bringing about a gale warning, 10 or 11 a storm warning (or "tropical storm warning" for 8 to 11 if related to a tropical cyclone), and anything stronger a hurricane warning.

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