Wind speed was not actually mentioned in the scale, but rather the force that was exerted on a Man-of-war ship. The descriptions for Beaufort numbers: 0 through to 4 describe the wind in terms of the speed that it may propel the ship. 5 through to 9 in terms of her mission and her sail carrying ability 10 through to 12 in terms of her survival.
The Original Beaufort Scale
1 1
Light Air
Or just sufficient to give steerage way.
2 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 3
Gentle Breeze
Or that in which a man-of-war with all sail set, and clean full would go in smooth water from.
3 to 4 knots
4 4
Moderate Breeze
Or that in which a man-of-war with all sail set, and clean full would go in smooth water from.
5 to 6 knots
5 5
Fresh Breeze
Or that to which a well-conditioned man-of-war could just carry in chase, full and by.
Royals, &c.
6 6
Strong Breeze
Or that to which a well-conditioned man-of-war could just carry in chase, full and by.
Single-reefed topsails and top-gal. sail
7 7
Moderate Gale
Or that to which a well-conditioned man-of-war could just carry in chase, full and by.
Double reefed topsails, jib, &c.
8 8
Fresh Gale
Or that to which a well-conditioned man-of-war could just carry in chase, full and by.
Treble-reefed topsails &c.
9 9
Strong Gale
Or that to which a well-conditioned man-of-war could just carry in chase, full and by.
Close-reefed topsails and courses.
10 10
Whole Gale
Or that with which she could scarcely bear close-reefed main- topsail and reefed fore-sail.
11 11
Storm
Or that which would reduce her to storm staysails.
12 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 transformed itself from a tool of the mariner to a means for the meteorologist.
The Modern Day Beaufort Scale
No.
Wind speed (knots)
World Meteorological Organization description
Sea state
Wave height (feet)
0 0
< 1
Calm
Calm; like a mirror
0
1 1
1 - 3
Light Air
Ripples with appearance of scales: no foam crests
½
2 2
4 - 6
Light Breeze
Small wavelets; crests of glassy appearance, not breaking
½ - 1
3 3
7 - 10
Gentle Breeze
Large wavelets; crests begin to break; scattered whitecaps
2 - 3
4 4
11 - 16
Moderate Breeze
Small waves, becoming longer numerous whitecaps
3½ - 5
5 5
17 - 21
Fresh Breeze
Moderate waves, taking longer form; many whitecaps; some spray
6 - 8
6 6
22 - 27
Strong Breeze
Larger waves forming; whitecaps everywhere; more spray
9½ - 13
7 7
28 - 33
Near gale
Sea heaps up; white foam from breaking waves begins to be blown in streaks
13½ - 19
8 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 9
41 - 47
Strong Gale
High waves; sea begins to roll; dense streaks of foam; spray may reduce visibility
23 - 32
10 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 11
56 - 63
Violent storm
Exceptionally high waves; sea covered with white foam patches; visibility still more reduced
39 - 46
12 12
>= 64
Hurricane
Air filled with foam; sea completely white with driving spray; visibility greatly reduced
37 - 52
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."