According to this page https://cumuluswiki.wxforum.net/a/Wind_ ... nd_Cumulus, wind speed could be multiplied with a factor of 1.25 if the anemometer is 2.5 meters above the ground.
But does anyone use this? What is right or what is wrong? Or is it just what you prefer?
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Latest Cumulus MX V3 release 3.28.6 (build 3283) - 21 March 2024
Cumulus MX V4 beta test release 4.0.0 (build 4019) - 03 April 2024
Legacy Cumulus 1 release 1.9.4 (build 1099) - 28 November 2014
(a patch is available for 1.9.4 build 1099 that extends the date range of drop-down menus to 2030)
Download the Software (Cumulus MX / Cumulus 1 and other related items) from the Wiki
Does anyone use wind measurement correction?
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Re: Does anyone use wind measurement correction?
Hi,
The "official" measurement of wind speed should be 10 metres above an "open field" with no obstructions, which is practically impossible for most amateur stations (and should be taken into account by most users of the data). If you are "posting" data which you hope to be useful, then generally the best that can be done is to state (on the website, or wherever) your specific measurement conditions (which would include the equipment used). Some people may put more weight on a "Davis" than a "FO" anemometer measurement, but either might be located particularly well or very badly.
There are some indications that the FO anemometer ALREADY over-reads by up to 20% (it has an unusually high quoted "Anemometer factor" of over 3), perhaps because the designers "realised" that it would rarely be located in an "ideal" location. However, Steve (the original author of Cumulus) maintained that his FO anemometer always UNDER-read in his location (the Orkneys, where wind speeds are very high). So perhaps the FO anemometer is not as linear (with wind speed) as some other designs? There is no facility to compensate for that in Cumulus (even assuming that you knew what compensation was required).
If your anemometer is located just a few metres above an "open field", then maybe it's worth applying a compensation factor. But normally there also will be obstructions in some directions that can produce turbulence and other variations, making results less meaningful. Also, the recommendation is NOT to compensate the "gust" value, so increasing the "average" value runs the risk of creating average wind speeds that are greater than the "maximum" gust !.
Cheers, Alan.
The "official" measurement of wind speed should be 10 metres above an "open field" with no obstructions, which is practically impossible for most amateur stations (and should be taken into account by most users of the data). If you are "posting" data which you hope to be useful, then generally the best that can be done is to state (on the website, or wherever) your specific measurement conditions (which would include the equipment used). Some people may put more weight on a "Davis" than a "FO" anemometer measurement, but either might be located particularly well or very badly.
There are some indications that the FO anemometer ALREADY over-reads by up to 20% (it has an unusually high quoted "Anemometer factor" of over 3), perhaps because the designers "realised" that it would rarely be located in an "ideal" location. However, Steve (the original author of Cumulus) maintained that his FO anemometer always UNDER-read in his location (the Orkneys, where wind speeds are very high). So perhaps the FO anemometer is not as linear (with wind speed) as some other designs? There is no facility to compensate for that in Cumulus (even assuming that you knew what compensation was required).
If your anemometer is located just a few metres above an "open field", then maybe it's worth applying a compensation factor. But normally there also will be obstructions in some directions that can produce turbulence and other variations, making results less meaningful. Also, the recommendation is NOT to compensate the "gust" value, so increasing the "average" value runs the risk of creating average wind speeds that are greater than the "maximum" gust !.
Cheers, Alan.
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Re: Does anyone use wind measurement correction?
In case this helps answer or otherwise informs this discussion, I post below the Wunderground pages from today for my 2 weather stations (I happen to have 2 running simultaneously at the moment as I'm in the process of changing over).
The first plots are from a Davis Vantage Vue mounted at about 1.5m above ground level. The second plots are from a (Maplin) Fine Offset mounted at about 2m above ground level, and about 2m away. Neither has any wind measurement correction applied. Please ignore the barometric pressures - I need to adjust the offset.
Davis Vantage Vue: Fine Offset: I see that:
The first plots are from a Davis Vantage Vue mounted at about 1.5m above ground level. The second plots are from a (Maplin) Fine Offset mounted at about 2m above ground level, and about 2m away. Neither has any wind measurement correction applied. Please ignore the barometric pressures - I need to adjust the offset.
Davis Vantage Vue: Fine Offset: I see that:
- The plots are generally remarkably similar
- The maximum temperature (at the vertical line) are 20.7 [Davis] and 20.5 [Fine Offset]
- The Dew point[ is reported higher from Davis [12.9] than Fine Offset [10.3]. The calculation for Dew Point of course depends on measured wind speed
- Wind gust is reported lower on Davis [33.8 km/h] than on Fine Offset [40.4 km/h] - ratio is 1.20
- Wind speed similarly lower on Davis [17.1 km/h] than on Fine Offset [21.2 km/h] - ratio is 1.24
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Last edited by ExperiMentor on Sat 28 Sep 2019 11:23 pm, edited 2 times in total.
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Re: Does anyone use wind measurement correction?
This is not true - dew point is independent of wind speed. The only way you could loosely relate dew point to wind speed would be in very light wind conditions where you could - potentially - have dew point influenced by previously-existing condensation inside the radiation screen by virtue of the evaporation of that condensation, and that extra water vapour not being removed from the radiation screen due to the lack of airflow through the screen.ExperiMentor wrote: ↑Fri 27 Sep 2019 7:52 pm
- The Dew point[ is reported higher from Davis [12.9] than Fine Offset [10.3]. The calculation for Dew Point of course depends on measured wind speed
- HansR
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Re: Does anyone use wind measurement correction?
Interesting. Both pieces of equipment at same installed height I assume? You could use a calibration with a handheld anemometer (of some quality). I would not just accept the Fine Offset as true (nor the Davis btw), why would you? And I assume you use the same units in both.ExperiMentor wrote: ↑Fri 27 Sep 2019 7:52 pm So, having considered all this, should I add a correction factor of +25% to the Davis wind speeds?
Hans
https://meteo-wagenborgen.nl
CMX build 4017+ ● RPi 3B+ ● Raspbian Linux 6.1.21-v7+ armv7l ● dotnet 8.0.3
https://meteo-wagenborgen.nl
CMX build 4017+ ● RPi 3B+ ● Raspbian Linux 6.1.21-v7+ armv7l ● dotnet 8.0.3
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- Posts: 214
- Joined: Tue 24 Nov 2015 11:30 pm
- Weather Station: Fine Offset & Davis Vantage Vue
- Operating System: Windows 10; Raspbian Buster
- Location: Switzerland
-
- Posts: 214
- Joined: Tue 24 Nov 2015 11:30 pm
- Weather Station: Fine Offset & Davis Vantage Vue
- Operating System: Windows 10; Raspbian Buster
- Location: Switzerland
Re: Does anyone use wind measurement correction?
The heights are stated (1.5m Davis and 2m Fine Offset), both much too low for 'correct' measurements (10m).HansR wrote: ↑Sat 28 Sep 2019 6:31 am Interesting. Both pieces of equipment at same installed height I assume? You could use a calibration with a handheld anemometer (of some quality). I would not just accept the Fine Offset as true (nor the Davis btw), why would you? And I assume you use the same units in both.
And of course I don't have an alternate anemometer.
The units are km/h for both.
My final comment was related to the preceding post suggesting that 20% might be added to readings from anemometers positoned too low, and that Fine Offset have this added by default. My values in a way seem to verify that Fine Offset have had such an addition, and there is rationale for it.
- HansR
- Posts: 5964
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- Location: Wagenborgen (NL)
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Re: Does anyone use wind measurement correction?
Yes, my bad.ExperiMentor wrote: ↑Sat 28 Sep 2019 11:23 pm The heights are stated (1.5m Davis and 2m Fine Offset), both much too low for 'correct' measurements (10m).
Understood. And yes, there is rationale.ExperiMentor wrote: ↑Sat 28 Sep 2019 11:23 pm My final comment was related to the preceding post suggesting that 20% might be added to readings from anemometers positoned too low, and that Fine Offset have this added by default. My values in a way seem to verify that Fine Offset have had such an addition, and there is rationale for it.
Got to read more precise with the two of you online
Btw: having calibration apparatus standby can be useful. I use one for P, T and RH. Wind is on my list.
Hans
https://meteo-wagenborgen.nl
CMX build 4017+ ● RPi 3B+ ● Raspbian Linux 6.1.21-v7+ armv7l ● dotnet 8.0.3
https://meteo-wagenborgen.nl
CMX build 4017+ ● RPi 3B+ ● Raspbian Linux 6.1.21-v7+ armv7l ● dotnet 8.0.3
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- Posts: 1124
- Joined: Sat 26 Feb 2011 1:58 pm
- Weather Station: Fine Offset 1080/1 & 3080
- Operating System: Windows XP SP3
- Location: SE London
Re: Does anyone use wind measurement correction?
Hi,
Normally, a "better" solution (to compensation) would be to mount the anemometer nearer to the "correct" height, which might also reduce air turbulence errors. But I believe the Davis equipment also mounts the Rain and T/H sensors in the same assembly, and if anything, these ideally should be mounted Lower. So, arguably, the FO design is rather "better" in that its cables allow each sensor to be mounted nearer to their ideal height.
For ExperiMentor's specific case, I would suggest that a "correction" factor IS introduced. But this would be as much to maintain "continuity" of his new (Davis) data from the original (Fine Offset) dataset, as for any perceived "absolute" accuracy.
Cheers, Alan.
Thanks for those interesting observations which appear to confirm that the OP shoould NOT introduce a compensation factor into his data (becasue it is already included in the Fine Offset anemometer - by accident or design).ExperiMentor wrote: ↑Fri 27 Sep 2019 7:52 pmSo, having considered all this, should I add a correction factor of +20% to the Davis wind speeds?
- Wind speed similarly lower on Davis [17.1 km/h] than on Fine Offset [21.2 km/h] - ratio is 1.24
Normally, a "better" solution (to compensation) would be to mount the anemometer nearer to the "correct" height, which might also reduce air turbulence errors. But I believe the Davis equipment also mounts the Rain and T/H sensors in the same assembly, and if anything, these ideally should be mounted Lower. So, arguably, the FO design is rather "better" in that its cables allow each sensor to be mounted nearer to their ideal height.
For ExperiMentor's specific case, I would suggest that a "correction" factor IS introduced. But this would be as much to maintain "continuity" of his new (Davis) data from the original (Fine Offset) dataset, as for any perceived "absolute" accuracy.
Cheers, Alan.