Wind measurement

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Wind observations are measured in two ways .... wind speed and wind direction. Where possible measuring devices should be in a large open area away from possible interference such as building, trees etc. It is generally accepted that measurements are based on readings at 10 metres (33 feet) above ground and the distance between the anemometer and any obstruction is at least ten times the height of the obstruction!


Wind Speed

Wind speed is measured using an anemometer. It is typically made up of cups attached to spindle. The stronger the wind the faster the cups rotate and the higher the reading. Several readings should be taken over a couple of minutes and an average taken. The measurement is sometimes made in knots but any speed unit is possible. Cumulus supports:

0 = m/s (metres per second)
1 = mph (miles per hour)
2 = km/h (kilometres per hour)
3 = kts (nautical knots)

Weather Stations and Cumulus

Compensation

As it says at FAQ#Where_should_I_position_the_wind_sensors the UK expects an anemonmeter to be 10m agl. Raising an anemometer to a height of 10 metres in an open space requires a strong mast with tensioned wires, and is not always possible, especially for weather enthusiasts, therefore one must build in a compensation figure (or multiplier) in order to take into account the height factor.

It is generally accepted that the following formula will provide a multiplier to compensate for lower height anemometer (where h is the height of the anemometer in metres above ground level).

= 1/(0.233 + 0.656*log10(h+4.75))

Examples:

Height (metres) Multiplier
0.5 1.42
1.0 1.37
1.5 1.32
2.0 1.29
2.5 1.25
3.0 1.22
3.5 1.20
4.0 1.18
4.5 1.15
5.0 1.13
5.5 1.12
6.0 1.10
6.5 1.08
7.0 1.07
7.5 1.06
8.0 1.04
8.5 1.03
9.0 1.02
9.5 1.01
10 1

Wind Multiplier in Cumulus

  • Within Cumulus it is possible to set a multiplier which will be used to upscale your actual reading from your anemometer to take into account its lower height. Use the formula above (or the table of examples) to locate your multiplier. Remember you are measuring the height of your anemometer from the ground in metres.
  • Within Cumulus click the 'Configuration' menu then 'Calibration'. Locate the 'Windspeed multiplier' and change this from its default of 1 to the value from above. Note Cumulus only accepts two decimal places if you have calculated using the formula rather than the table. You should also ask Cumulus to calculate wind chill, because any value output by your weather station will be based on an uncompensated wind speed.
  • In addition to a Wind Speed multiplier, Cumulus allows for a Wind Gust multiplier. However, it is generally felt that wind gusts should not take into account their height from ground level therefore a multiplier of 1 is acceptable.

Beaufort Scale

The Beaufort Scale was developed as a common way to measure and describe wind speed prior to any accurate devices. Initially it was based on the condition of the sea however the modern Beaufort scale incorporates land observations too. Note that the scale applies to sustained (average) wind speeds.

Using the table below you could asses if your Calibration within Cumulus is roughly accurate based on observations.

Beaufort number Description Wind speed Land conditions
km/h mph kts m/s
0 Calm < 1 < 1 < 1 < 0.3 Calm. Smoke rises vertically.
1 Light air 1 – 5 1 – 3 1 – 2 0.3 – 1.5 Wind motion visible in smoke.
2 Light breeze 6 – 11 3 – 7 3 – 6 1.5 – 3.3 Wind felt on exposed skin. Leaves rustle.
3 Gentle breeze 12 – 19 8 – 12 7 – 10 3.3 – 5.5 Leaves and smaller twigs in constant motion.
4 Moderate breeze 20 – 28 13 – 17 11 – 15 5.5 – 8.0 Dust and loose paper raised. Small branches begin to move.
5 Fresh breeze 29 – 38 18 – 24 16 – 20 8.0 – 11 Branches of a moderate size move. Small trees begin to sway.
6 Strong breeze 39 – 49 25 – 30 21 – 26 11 – 14 Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic garbage cans tip over.
7 High wind, Moderate gale, Near gale 50 – 61 31 – 38 27 – 33 14 – 17 Whole trees in motion. Effort needed to walk against the wind. Swaying of skyscrapers may be felt, especially by people on upper floors.
8 Gale, Fresh gale 62 – 74 39 – 46 34 – 40 17 – 20 Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.
9 Strong gale 75 – 88 47 – 54 41 – 47 21 – 24 Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over. Damage to circus tents and canopies.
10 Storm, Whole gale 89 – 102 55 – 63 48 – 55 25 – 28 Trees are broken off or uprooted, saplings bent and deformed. Poorly attached asphalt shingles and shingles in poor condition peel off roofs.
11 Violent storm 103 – 117 64 – 72 56 – 63 29 – 32 Widespread damage to vegetation. Many roofing surfaces are damaged; asphalt tiles that have curled up and/or fractured due to age may break away completely.
12 Hurricane ≥ 118 ≥ 73 ≥ 64 ≥ 33 Very widespread damage to vegetation. Some windows may break; mobile homes and poorly constructed sheds and barns are damaged. Debris may be hurled about.

Forces 13 to 17 have been included widely in the past, but are not part of the current international scale.

Cumulus can track force 8 and stronger days - see Gale_Days

Wind Direction

Wind direction, like speed, should ideally be measured in a large open space away from buildings etc which could provide shelter or help funnel winds.

Wind direction is measured using a wind vane. This is ideally mounted on a roof top or mast 10 metres above ground. The wind vane is made up of an arrow mounted on top of a spindle, allowing the arrow to move with the wind. The arrow always points in the direction from which the wind is blowing and similarly wind direction is reported as from where the wind is blowing. Example, a Southerly wind is wind coming from the South

Wind direction is typically described in Cardinal points but Cumulus uses bearing (integer in angular degrees) in its logs.