The initial scale of thirteen classes (zero to twelve) did not reference wind speed numbers, but related qualitative wind conditions to effects on the sails of a man of war, then the main ship of the Royal Navy, from "just sufficient to give steerage" to "that which no canvas sails could withstand." At zero, all his sails would be up; at six, half of his sails would have been taken down; and at twelve, all sails would be stowed away.
The scale was made a standard for ship's log entries on Royal Navy vessels in the late 1830s, and was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. In 1906, to accommodate the growth of steam power, the descriptions were changed to how the sea, not the sails, behaved and extended to land observations. Rotations to scale numbers were standardized only in 1923. George Simpson, Director of the UK Meteorological Office, was responsible for this and for the addition of the land-based descriptors. The measure was slightly altered some decades later to improve its utility for meteorologists. Today, many countries have abandoned the scale and use the SI-based units m/s or km/h instead, but the severe weather warnings given to public are still approximately the same as when using the Beaufort scale.
The Beaufort scale was extended in 1946, when Forces 13 to 17 were added. However, Forces 13 to 17 were intended to apply only to special cases, such as tropical cyclones. Nowadays, the extended scale is only used in Taiwan and mainland China, which are often affected by typhoons.
Wind speed on the 1946 Beaufort scale is based on the empirical formula:
where v is the equivalent wind speed at 10 metres above the surface and B is Beaufort scale number. For example, B = 9.5 is related to 24.5 m/s which is equal to the lower limit of "10 Beaufort". Using this formula the highest winds in hurricanes would be 23 in the scale.
Today, hurricane force winds are sometimes described as Beaufort scale 12 through 16, very roughly related to the respective category speeds of the Saffir-Simpson Hurricane Scale, by which actual hurricanes are measured, where Category 1 is equivalent to Beaufort 12. However, the extended Beaufort numbers above 13 do not match the Saffir-Simpson Scale. Category 1 tornadoes on the Fujita and TORRO scales also begin roughly at the end of level 12 of the Beaufort scale but are indeed independent scales.
Note that wave heights in the scale are for conditions in the open ocean, not along shore.
|Beaufort number||Wind speed||Description||Wave height||Sea conditions||Land conditions||Sea state photo|
|0||<1||<1||<1||<0.3||Calm||0||0||Flat.||Calm. Smoke rises vertically.|
|1||1-5||1-3||1-2||0.3-1.5||Light air||0.1||0.33||Ripples without crests.||Wind motion visible in smoke.|
|2||6-11||3-7||3-6||1.5-3.3||Light breeze||0.2||0.66||Small wavelets. Crests of glassy appearance, not breaking||Wind felt on exposed skin. Leaves rustle.|
|3||12-19||8-12||7-10||3.3-5.5||Gentle breeze||0.6||2||Large wavelets. Crests begin to break; scattered whitecaps||Leaves and smaller twigs in constant motion.|
|4||20-28||13-17||11-15||5.5-8.0||Moderate breeze||1||3.3||Small waves.||Dust and loose paper raised. Small branches begin to move.|
|5||29-38||18-24||16-20||8.0-10.8||Fresh breeze||2||6.6||Moderate (1.2 m) longer waves. Some foam and spray.||Branches of a moderate size move. Small trees begin to sway.|
|6||39-49||25-30||21-26||10.8-13.9||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. Empty plastic garbage cans tip over.|
|7||50-61||31-38||27-33||13.9-17.2||High wind, Moderate Gale, Near Gale||4||13.1||Sea heaps up and foam begins to streak.||Whole trees in motion. Effort needed to walk against the wind. Swaying of skyscrapers may be felt, especially by people on upper floors.|
|8||62-74||39-46||34-40||17.2-20.7||Fresh Gale||5.5||18||Moderately high waves with breaking crests forming spindrift. Streaks of foam.||Twigs broken from trees. Cars veer on road.|
|9||75-88||47-54||41-47||20.7-24.5||Strong Gale||7||23||High waves (6-7 m) with dense foam. Wave crests start to roll over. Considerable spray.||Larger branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over. Damage to circus tents and canopies.|
|10||89-102||55-63||48-55||24.5-28.4||Whole Gale/Storm||9||29.5||Very high waves. 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.||Trees are broken off or uprooted, saplings bent and deformed, poorly attached asphalt shingles and shingles in poor condition peel off roofs.|
|11||103-117||64-72||56-63||28.4-32.6||Violent storm||11.5||37.7||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.||Widespread vegetation damage. More damage to most roofing surfaces, asphalt tiles that have curled up and/or fractured due to age may break away completely.|
|12||≥118||≥73||≥64||≥32.6||Hurricane-force||≥14||≥46||Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.||Considerable and widespread damage to vegetation, a few windows broken, structural damage to mobile homes and poorly constructed sheds and barns. Debris may be hurled about.|
This scale is also widely used in China, Taiwan, Hong Kong and Macau, however with some differences between them. Taiwan uses the Beaufort scale with the extension to 17 noted above. China also switched to this extended version without prior notice on the morning of May 15, 2006, and the extended scale was immediately put to use for Typhoon Chanchu. Hong Kong and Macau however keep using Force 12 as the maximum.
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 to 12 a hurricane warning.