|
Beaufort |
Specification of Beaufort scale for uses on land, based on observations made at land stations |
Mean pressure at standard density
on a disk of 1 ft2 |
Gust speeds |
Speed |
Limits of speed in open at
33 ft. |
|
Calm: smoke rises vertically |
0 |
0 |
0 |
0-0.2 |
|
Light air: Direction of wind shown by smoke drift, but not by wind vanes |
0.01 |
1-6 |
1-5 |
0.3-1.5 |
|
Light breeze: Wind felt on face; leaves rustle; ordinary vane moved by wind |
0.04 |
7-14 |
6-11 |
1.6-3.3 |
|
Gentle breeze: Leaves and small twigs in constant motion; wind extends light flag |
0.13 |
15-24 |
12-19 |
3.4-5.4 |
|
Moderate breeze: Raises dust and loose paper; small branches are moved |
0.32 |
25-35 |
20-28 |
5.5-7.9 |
|
Fresh breeze: Small trees in leaf begin to sway; crested wavelets form on inland waters |
0.62 |
36-48 |
29-38 |
8.0-10.7 |
|
Strong breeze: Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty |
1.1 |
49-61 |
39-49 |
10.8-13.8 |
|
Near gale: Whole trees in motion; inconvenience felt when walking against wind |
1.7 |
62-76 |
50-61 |
13.9-17.1 |
|
Gale: Breaks twigs off trees; generally impedes progress |
2.6 |
77-93 |
62-74 |
17.2-20.7 |
|
Strong gale: Slight structural damage occurs [chimney pots and slates removed] |
3.7 |
94-110 |
75-88 |
20.8-24.4 |
|
Storm: Seldom experienced inland; trees uprooted; considerable structural damage occurs |
5.0 |
111-129 |
89-103 |
24.5-28.4 |
|
Violent storm: Very rarely experienced; accompanied by widespread damage |
6.7 |
130-146 |
104-117 |
28.5-32.6 |
|
Hurricane: … … … … |
>8.1 |
>146 |
>117 |
>32.7 |
The Beaufort scale was devised by Admiral Sir Francis Beaufort in 1805, and became mandatory for log entries for all ships of the British Royal Navy in 1838. In 1874 it was adopted in modified form for international maritime use. Subsequently somewhat different versions of the descriptions were adopted for use on land. Originally the scale made no reference to the numerical speed of the wind, but these were introduced early in the 20th century. In the table used here, these speeds in km/h are equivalents for an anemometer at a height of 6 m [Crowder, 1995; Meteorological Office, 1963].
The specification of the steps of the scale originally had reference to a fully-rigged man-of-war in the period 1800-1850. The velocity equivalents given in the table are based on the empirical relationship between estimated number and measured velocity, V = 1.87 Ö B3, where V is in miles per hour, and B is the corresponding Beaufort number. The pressure equivalents are derived from the relationship p = 0.003V2, where p is in pounds per square foot and V is in miles per hour [Meteorological Office, 1963].
The gust speeds in the table above have been estimated by multiplying wind speeds by 1.25; i.e. it is assumed that the Beaufort wind speed are sustained 1-minute wind speeds. The multiplication factor is from Lander and Guard [1991].
The mean wind speeds given here are the same as those in Munich Re (1988, p53) though the latter are stated to be for a height of 10m above the surface. The wind speeds given in UNDRO (1991, p47) are lower then the mean wind speeds given here, but the Specifications are the same!
* The pressure due to the wind on any object exposed to it arises from the impact of the air on the windward side and suction on the leeward side. The mean pressure depends on the size and shape of the object. The values given are for a disk of 1 ft2 in area, but they apply with fair approximation for circular or square plates from 1 ft2 to 10 ft2 in area. 1 mb- 10kg/m2
References
Crowder B, 1995, The wonders of the weather, Bureau of Meteorology, Australian Government Publishing service, Canberra 270p.
Meteorological Office, 1963, Meteorological Glossary, Her Majesty's Stationery Office, London, 4th edition, 288p.
Munich Re, 1998, World Map of Natural Hazards, Munich Re, Munich, 53p.
Lander, M. A. & Guard, C. P., 1191, "Towards a consensus and public understanding of the definitions of maximum sustained winds in tropical cyclones, Proceedings of the Second Workshop for Natural Disaster Reduction, Japan-U.S. Science & Technology Agreement, Karuizaira, Nagono, Japan, 23-27 September, 1991, 420-422
