Beaufort scale

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Force 12 at sea.
Force 12 at sea.

The Beaufort scale (pronounced /ˈboʊfət/) is an empirical measure for describing wind speed based mainly on observed sea conditions. Its full name is the Beaufort wind force scale.

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[edit] History

The scale was created in 1805 by Sir Francis Beaufort, an Irish-British admiral and hydrographer. The scale that carries Beaufort's name had a long and complex evolution, from the previous work of others, to when Beaufort was a top administrator in the Royal Navy in the 1830s. In the early 19th Century naval officers made regular weather observations, but there was no standard scale and so they could be very subjective - one man's "stiff breeze" might be another's "soft breeze". Beaufort succeeded in getting things standardized.

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.[1]

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.[2] 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,[citation needed] 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:[3]

v = 0.836 B3/2 m/s

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, hurricanes are sometimes described as Beaufort scale 12 through 16, very roughly related to the standard Saffir-Simpson Hurricane Scale where Category 1 is equivalent to Beaufort 12. However, the Saffir-Simpson Scale does not match the extended Beaufort numbers above 13. 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.

[edit] The modern scale

Beaufort number Wind speed Mean wind speed (kn / km/h / mph) Description Wave height Sea conditions Land conditions Sea state photo
kn km/h mph m/s m ft
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 11-15 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 16-21 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/Moderate gale 4 13.1 Sea heaps up and foam begins to streak. Whole trees in motion. Effort needed to walk against the wind.
8 34-40 63-75 39-46 17.2-20.7 37 / 68 / 42 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 41-47 76-87 47-54 20.8-24.4 44 / 81 / 50 Strong Gale 7 23 High waves (6-7 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 Whole Gale/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 56-63 103-119 64-73 28.5-32.6 60 / 112 / 70 Violent storm 11.5 37.7 Exceptionally high waves. Widespread structural damage.
12 64-80 120 74-95 32.7-40.8 73 / 148 / 90 Hurricane 14+ 46+ Huge waves. Air filled with foam and spray. Sea completely white with driving spray. Visibility greatly reduced. Considerable 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, 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[4], 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.

[edit] References

  • Huler, Scott (2004). Defining the Wind: The Beaufort Scale, and How a 19th-Century Admiral Turned Science into Poetry. Crown. ISBN 1-4000-4884-2. 
  1. ^ BBC - Weather Centre - Features - Understanding Weather - Beaufort Scale
  2. ^ Met Office: The Beaufort scale
  3. ^ Tom Beer (1997). Environmental Oceanography. CRC Press. ISBN 0849384257. 
  4. ^ 昨日实行新标准“珍珠”属强台风_新闻中心_新浪网

[edit] See also

[edit] External links