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Wind measurement: Difference between revisions
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=Wind Speed= | =Wind Speed= | ||
Wind speed is measured using an [http://en.wikipedia.org/wiki/Anemometer 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 | Wind speed is measured using an [http://en.wikipedia.org/wiki/Anemometer 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 [http://en.wikipedia.org/wiki/Knot_%28unit%29 knots] but any speed unit is possible. | ||
Cumulus [[Cumulus.ini_(Cumulus_1)|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 == | |||
*If you have a Davis weather station see [[FAQ#What_do_the_various_wind_speeds_in_Cumulus_1_correspond_to.2C_with_Davis_stations.3F|FAQ: What do the various wind speeds in Cumulus correspond to with Davis stations]] | |||
*If you have a Fine Offset station, see [[FAQ#What_do_the_various_wind_speeds_in_Cumulus_1_correspond_to.2C_with_Fine_Offset_stations.3F|FAQ: What do the various wind speeds in Cumulus correspond to with Fine Offset stations]] | |||
* For Oregon Scientific and La Crosse stations see [[FAQ#How_do_I_set_up_recording_wind_speeds_in_Cumulus_1.2C_with_Oregon_Scientific_and_La_Crosse_stations| FAQ: How do I set up recording wind speeds in Cumulus with Oregon Scientific and La Crosse stations]] | |||
==Compensation== | ==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 <tt>h</tt> is the height of the anemometer in metres above ground level). | |||
It is generally accepted that the following formula will provide a multiplier to compensate for lower height anemometer. | |||
= 1/(0.233 + 0.656*log10(h+4.75)) | = 1/(0.233 + 0.656*log10(h+4.75)) | ||
===Examples:=== | |||
Examples: | |||
{| class="wikitable" | {| class="wikitable" | ||
!|Height (metres) || Multiplier | !|Height (metres) || Multiplier | ||
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|- | |- | ||
|} | |} | ||
===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. | |||
==Wind Multiplier in Cumulus== | *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. | |||
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 | |||
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. | |||
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== | ==Beaufort Scale== | ||
The [http://en.wikipedia.org/wiki/Beaufort_scale 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. | |||
The [http://en.wikipedia.org/wiki/Beaufort_scale 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. | |||
Using the table below you could asses if your Calibration within Cumulus is roughly accurate based on observations. | Using the table below you could asses if your Calibration within Cumulus is roughly accurate based on observations. | ||
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| 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. | | 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= | ||