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Does anyone use wind measurement correction?

Posted: Tue 17 Sep 2019 7:49 am
by nowind
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?

Re: Does anyone use wind measurement correction?

Posted: Sat 21 Sep 2019 3:00 pm
by AllyCat
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.

Re: Does anyone use wind measurement correction?

Posted: Fri 27 Sep 2019 7:52 pm
by ExperiMentor
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:
Davis.jpg
Fine Offset:
FineOffset.jpg
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
So, having considered all this, should I add a correction factor of +20% to the Davis wind speeds?

Re: Does anyone use wind measurement correction?

Posted: Fri 27 Sep 2019 8:25 pm
by freddie
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
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.

Re: Does anyone use wind measurement correction?

Posted: Sat 28 Sep 2019 6:31 am
by HansR
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?
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.

Re: Does anyone use wind measurement correction?

Posted: Sat 28 Sep 2019 11:14 pm
by ExperiMentor
freddie wrote:
Fri 27 Sep 2019 8:25 pm
This is not true - dew point is independent of wind speed.
My mistake. The discrepancy will be due to a substantial difference in Humidity measured by the 2 devices, and not plotted by Wunderground.

Re: Does anyone use wind measurement correction?

Posted: Sat 28 Sep 2019 11:23 pm
by ExperiMentor
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.
The heights are stated (1.5m Davis and 2m Fine Offset), both much too low for 'correct' measurements (10m).
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.

Re: Does anyone use wind measurement correction?

Posted: Sun 29 Sep 2019 8:04 am
by HansR
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).
Yes, my bad.
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.
Understood. And yes, there is rationale.

Got to read more precise with the two of you online :oops:

Btw: having calibration apparatus standby can be useful. I use one for P, T and RH. Wind is on my list.

Re: Does anyone use wind measurement correction?

Posted: Sun 29 Sep 2019 8:34 pm
by AllyCat
Hi,
ExperiMentor wrote:
Fri 27 Sep 2019 7:52 pm
  • Wind speed similarly lower on Davis [17.1 km/h] than on Fine Offset [21.2 km/h] - ratio is 1.24
So, having considered all this, should I add a correction factor of +20% to the Davis wind speeds?
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).

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.