Wind Information
Wind Speeds and Loads.
The wind blowing on an object causes pressure, uplift and suction on various of the objects surfaces. The pressure created against the structure changes on a non-linear basis, As the wind speed increases, the pressure increases more quickly. As an example, the pressure caused by a wind speed of 57mph is 25 times higher than the pressure caused by a wind speed of 10mph. These pressures are usually increased again by the effects of the wind accelerating around objects.
As we see more examples of high winds during the summer event season it becomes more important to design structures according to the latest standards. The standards are updated regularly as engineers learn more about the effects of the wind. Consequently, to ensure safety, structures should be designed using the current version of BS6399, Part 2. Structures designed to the now obsolete "CP3" code are unlikely to be satisfactory. All our structures are fully designed to take all effects of wind forces into account, based on the latest version of BS6399 Part 2.
The Beaufort Scale for estimating wind speed by observation is named after Sir Admiral F. Beaufort (1774-1857). It was more use in the days before simple and accurate anemometers were available. This version uses descriptions more suited to observations that can be made on land, and provides useful conversion to different units of measurement, coupled with the pressures caused. It is interesting but is still no substitute for the accurate electronic anemometers fitted to every single one of our stages and towers.
For convenience we have converted the Beaufort ratings into other units in the table below. The last column gives the pressure caused by wind of that speed. This is in kiloNewtons per square metre. Very roughly 1 kiloNewton is equivalent to 100 kgs.
| Beaufort Force | Description | Specification on land | Knots | km/h | mph | m/s | kN/m2 |
| 0 | Calm | Smoke rises vertically | 0 | 0 | 0 | ||
| 1 | Very light | Direction of wind shown by smoke drift but not by wind vanes | 1-3 | 1-5 | 1-3 | 1-2 | 0.002 |
| 2 | Light Breeze | Wind felt on face, leaves rustle, ordinary wind vane moved by wind | 4-6 | 6-11 | 4-7 | 2-3 | 0.005 |
| 3 | Gentle Breeze | Leaves and small twigs in constant motion, wind extends light flag | 7-10 | 12-19 | 8-12 | 3-5 | 0.015 |
| 4 | Moderate breeze | Wind raises dust and loose paper, small branches move | 11-16 | 20-29 | 13-18 | 5-8 | 0.039 |
| 5 | Fresh breeze | Small trees in leaf start to sway | 17-21 | 30-39 | 19-24 | 8-11 | 0.074 |
| 6 | Strong breeze | Large branches in motion, telegraph wires whistle | 22-27 | 40-50 | 25-31 | 11-14 | 0.120 |
| 7 | Near gale | Whole trees in motion, inconvenient to walk against wind | 28-33 | 51-61 | 32-38 | 14-17 | 0.177 |
| 8 | Gale | Twigs break from trees, difficult to walk | 34-40 | 62-74 | 39-46 | 17-20 | 0.245 |
| 9 | Strong gale | Slight structural damage occurs, chimney pots and slates removed | 41-47 | 75-87 | 47-54 | 20-24 | 0.353 |
| 10 | Storm | Trees uprooted, considerable structural damage | 48-55 | 88-101 | 55-63 | 24-28 | 0.481 |
| 11 | Violent storm | Widespread damage | 56-63 | 102-117 | 64-73 | 28-32 | 0.628 |
| 12 | Hurricane | Widespread damage | >64 | >118 | >74 | >32 |
Wind Chill Equivalent Temperatures (Steadman)
Wind chill is a serious problem for crews working on winter events. In the table below, the left hand column gives the temperature a sheltered thermometer would record. By reading across the top to find the wind speed being measured on site (multiply knots by 1.1 to get mph OR divide knots by 2 to get m/s), the table gives the effective temperature experienced by a body exposed to the wind.
| Temp °C | Wind Speed at 10m above ground (Knots) | |||||||
| 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | |
| 20 | 19.1 | 17.4 | 15.9 | 14.9 | 14.0 | 13.3 | 12.8 | 12.3 |
| 18 | 17.0 | 15.2 | 13.7 | 12.5 | 11.5 | 10.8 | 10.2 | 9.7 |
| 16 | 14.9 | 13.0 | 11.4 | 10.1 | 9.0 | 8.2 | 7.6 | 7.0 |
| 14 | 12.9 | 10.8 | 9.1 | 7.6 | 6.5 | 5.6 | 4.9 | 4.2 |
| 12 | 10.8 | 8.6 | 6.7 | 5.2 | 4.0 | 3.0 | 2.1 | 1.4 |
| 10 | 8.7 | 6.4 | 4.4 | 2.7 | 1.4 | 0.2 | -0.6 | -1.4 |
| 8 | 6.7 | 4.2 | 2.0 | 0.2 | -1.2 | -2.5 | -3.4 | -4.2 |
| 6 | 4.6 | 2.0 | -0.4 | -2.3 | -3.9 | -5.2 | -6.3 | -7.0 |
| 4 | 2.5 | -0.3 | -2.8 | -4.8 | -6.5 | -7.9 | -9.1 | -10.0 |
| 2 | 0.4 | -2.5 | -5.2 | -7.3 | -9.1 | -10.7 | -11.9 | -12.9 |
| 0 | -1.7 | -4.8 | -7.5 | -9.9 | -11.8 | -13.3 | -14.6 | -15.8 |
| -2 | -3,7 | -7.1 | -9.9 | -12.3 | -14.4 | -16.1 | -17.4 | -18.6 |
| -4 | -5.8 | -9.3 | -12.3 | -14.8 | -17.0 | -18.8 | -20.2 | -21.4 |
| -6 | -7.9 | -11.6 | -14.6 | -17.3 | -19.6 | -21.3 | -22.9 | -24.2 |
| -8 | -10.0 | -13.9 | -17.0 | -19.9 | -22.2 | -24.0 | -25.6 | -27.0 |
| -10 | -12.1 | -16.1 | -19.4 | -22.4 | -24.7 | -26.6 | -28.3 | -29.8 |
| -12 | -14.2 | -18.3 | -21.7 | -24.9 | -27.3 | -29.3 | -31.0 | -32.6 |
| -14 | -16.3 | -20.6 | -24.1 | -27.3 | -29.9 | -31.9 | -33.8 | -35.3 |
| -16 | -18.3 | -22.8 | -26.5 | -29.7 | -32.4 | -34.6 | -36.5 | -38.0 |
| -18 | -20.4 | -25.0 | -28.9 | -32.2 | -34.9 | -37.2 | -39.1 | -40.8 |
| -20 | -22.5 | -27.2 | -31.2 | -34.7 | -37.4 | -39.7 | -41.7 | -43.5 |