Beaufort Scale

The Beaufort wind force scale used to indicate weather conditions is well known to anyone who has heard the BBC's shipping forecast. The original scale as created by Sir Francis Beaufort related to what he saw as the different states of sea condition which affected a sailing warship - a "well-conditioned man-of-war."

Wind speed was not actually mentioned in the scale, but rather the force that was exerted on a Man-of-war. The descriptions for Beaufort numbers 0 through to 4 describe the wind in terms of the speed that it may propel the ship; those for 5 through to 9 in terms of her mission and her sail carrying ability; and those for 10 through to 12 in terms of her survival.

The Original Beaufort Scale

1 Light Air Or just sufficient to give steerage way.
2 Light Breeze Or that in which a man-of-war with all sail set, and clean full would go in smooth water from. 1 to 2 knots
3 Gentle Breeze 3 to 4 knots
4 Moderate Breeze 5 to 6 knots
5 Fresh Breeze Or that to which a well-conditioned man-of-war could just carry in chase, full and by. Royals, &c.
6 Strong Breeze Single-reefed topsails and top-gal. sail
7 Moderate Gale Double reefed topsails, jib, &c.
8 Fresh Gale Treble-reefed topsails &c.
9 Strong Gale Close-reefed topsails and courses.
10 Whole Gale Or that with which she could scarcely bear close-reefed main-topsail and reefed fore-sail.
11 Storm Or that which would reduce her to storm staysails.
12 Hurricane Or that which no canvas could withstand.

The Royal Navy made Beaufort's scale mandatory in 1838 but it wasn't until 1912 that the International Commission for Weather Telegraphy sought some agreement on velocity equivalents for the Beaufort scale. A uniform set of equivalents was accepted in 1926 and revised slightly in 1946. By 1955, wind velocities in knots replaced Beaufort numbers on weather maps. But there were still a need for eyeball estimates by seamen to fill the gaps in the global observing network. Thus it became imperative to relate the seaman's guess logged in Beaufort numbers to the wind speed in knots. And so Beaufort's scale had transfomed itself from a tool of the mariner to a means for the meteorologist.

The Modern Day Beaufort Scale
Beaufort
number
Wind Speed
(knots)
World Meteorological Organization description Sea state: Wave height (feet):
0 < 1 Calm Calm; like a mirror 0
1 1 - 3 Light air Ripples with appearance of scales: no foam crests
2 4 - 6 Light breeze Small wavelets; crests of glassy appearance, not breaking - 1
3 7 - 10 Gentle breeze Large wavelets; crests begin to break; scattered whitecaps 2 - 3
4 11 - 16 Moderate breeze Small waves, becoming longer numerous whitecaps 3 - 5
5 17 - 21 Fresh breeze Moderate waves, taking longer form; many whitecaps; some spray 6 - 8
6 22 - 27 Strong breeze Larger waves forming; whitecaps everywhere; more spray 9 - 13
7 28 - 33 Near gale Sea heaps up; white foam from breaking waves begins to be blown in streaks 13 - 19
8 34 - 40 Gale Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks 18 - 28
9 41 - 47 Strong gale High waves; sea begins to roll; dense streaks of foam; spray may reduce visibility 23 - 32
10 48 - 55 Storm Very high waves with overhanging crests; sea takes white appearance as foam is blown in very dense streaks; rolling is heavy and visibility is reduced 29 - 41
11 56 - 63 Violent storm Exceptionally high waves; sea covered with white foam patches; visibility still more reduced 39 - 46
12 >= 64 Hurricane Air filled with foam; sea completely white with driving spray; visibility greatly reduced 37 - 52