The Beaufort Scale
The Beaufort Scale is a convenient way to describe the force of a wind. It describes winds by their speed in nautical knots or miles per hour, a short descriptive term describing its force, such as 'Light Breeze', a more detailed land or sea state description, and a convenient Beaufort Number, ranging from 0 to 12. Wind speeds are measured at 10m (33 feet) above ground level (or sea level), so they will often seem a little less at ground level.
Francis Beaufort was born in 1774 in Ireland and began his career in the Royal Navy when he was 13, eventually becoming Rear Admiral Sir Francis Beaumont, Knight Commander of the Bath, and being the Hydrographer to the Admiralty. In the summer of 1805, whilst the commander of the Woolwich, a 44 gun man-of-war, he invented his wind force scale, although the Royal Navy didn't adopt his method until1838.
Early copies of the scale contained descriptions that are now meaningless to most of us because we don't sail in wooden fighting ships, but the scale remains with updated descriptions suited to different conditions around the world. An early version of the scale is shown below.
| 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 reason for having so many ways to describe the wind is that it is, of course, invisible. Unless you have a mechanical way to measure the speed of the wind, its speed can only be estimated by looking at its effects on your surroundings. The descriptive scale is liable to revision in different parts of the world. Just as 'trees uprooted' is no use at all when you are in the middle of an ocean, 'leaves and small twigs in constant motion - wind can extend a lightweight flag.' is no use when you live in a desert and don't own a flag!
Beaufort Number |
Speed in knots |
Speed in miles per hour |
Descriptive term |
Effect
on Land |
Effect
at Sea |
0 |
Less than 1 |
Less than 1 |
Calm |
Smoke rises vertically. |
Sea like
a mirror. |
1 |
1 to 3 |
1 to 3 |
Light Air |
Direction of wind shown by smoke but not wind vanes. |
Ripples with the appearance ofscales are formed, but withoutfoam crests. |
2 |
4 to 6 |
4 to 7 |
Light Breeze |
Leaves rustle,
wind felt on face and wind vanes moved by wind |
Small wavelets, still short,but more pronounced. Crestshave a glassy appearance anddo not break. |
3 |
7 to 10 |
8 to 12 |
Gentle Breeze |
Leaves and small twigs in constant motion. Wind can
extend a lightweight flag. |
Large wavelets. Crests beginto break. Foam of glassyappearance. Perhaps scatteredwhite horses. |
4 |
11 to 16 |
13 to 18 |
Moderate Breeze |
Moves small branches and will raise dust and loose
paper. |
Small waves, becoming larger; fairly frequent white horses. |
5 |
17 to 21 |
19 to 24 |
Fresh Breeze |
Small trees in leaf will begin to sway. |
Moderate waves, taking a more pronounced long form; manywhite horses are formed. |
6 |
22 to 27 |
25 to 31 |
Strong Breeze |
Whistling can be heard in phone (telegraph) wires.
Large branches are in motion.
Umbrellas used with difficulty. |
Large waves begin to form; thewhite foam crests are moreextensive everywhere.Probably some spray. |
7 |
28 to 33 |
32 to 38 |
Near Gale |
Whole trees in motion. Inconvenience felt when walking
against the wind. |
Sea heaps up and white foamfrom breaking waves begins tobe blown in streaks along thedirection of the wind. |
8 |
34 to 40 |
39 to 46 |
Gale |
Breaks twigs off trees and progress is impeded. |
Moderately high waves of greater length; edges of crests begin tobreak into spindrift. The foam isblown in well-marked streaksalong the direction of the wind. |
9 |
41 to 47 |
47 to 54 |
Strong Gale |
Slight structural damage occurs. Chimney pots and
roof slates are blown off. |
High waves. Dense streaks offoam along the direction of thewind. Crests of waves begin totopple, tumble and roll over.Spray may affect visibility. |
10 |
48 to 55 |
55 to 63 |
Storm |
Trees uprooted. Considerable structural damage occurs. |
Very high waves with overhanging crests. Great patches
of foam blown in dense white streaks. Visibility affected. |
11 |
56 to 63 |
64 to 72 |
Violent Storm |
Widespread damage to buildings. Rarely experienced
in Britain. |
Exceptionally high waves (smaller ships may vanish
from sight behind waves) and
the sea is totally covered with long white patches of foam lying along
the direction of the wind. Visibility seriously affected.X |
12 |
64 to 71 |
73 to 83 |
Hurricane |
Only experienced in the tropics. |
Only experienced in the tropics.The air is filled
with foam and spray.
Sea completely white
with driving spray;visibility very seriously
affected. |
 |
 |
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