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wind speed correction

Discussion and questions about Cumulus weather station software version 1. This section is the main place to get help with Cumulus 1 software developed by Steve Loft that ceased development in November 2014.
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mikentl
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wind speed correction

Post by mikentl »

Hi all
Would someone please explane to me the difference between putting a number in the Wind gauge range, and putting a multplier in the wind speed calibration box.
I normally have a multiplier of 1.5 in the calibration box as the only position i can put the sensor is slightly sheltered.
The surrounding trees are showing a much stronger wind than the sensor shows, the multiplier seems to correct this.
If I am doing this all wrong perhaps someone will put me on the right track.
FineOffset WH1080 mounted 15ft high in back garden with a 20yds clearance from buildings and trees about 70yds away.
Many thanks Mike
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steve
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Re: wind speed correction

Post by steve »

What you're doing is correct. The calibration value is to compensate for situations like yours; it alters the value of the recorded data.

The 'wind gauge range' is purely a display option to set the initial maximum value of the wind speed gauge on the Cumulus main screen. As it says in the help:

Wind gauge range This allows you to select the initial range for the wind gauge. When the wind speed gets above this value, the range will increase to 2x the value, then 3x, and so on. The minimum value you can supply is 5. Note that some values will cause the gauge scale to look rather odd; multiples of 5 are probably best.
Steve
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daj
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Re: wind speed correction

Post by daj »

This is very interesting and timely for my problem....I have felt for a while that my wind speeds are too low. I've been thinking of re-siting the instruments but I am really stuck as the garden is surrounded by huge trees; over 100 years old and twice the height of my house (which is three storeys).

how do I estimate a multiplier?

thanks
David
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steve
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Re: wind speed correction

Post by steve »

daj wrote:how do I estimate a multiplier?
It's very difficult. There is a 'standard' formula for compensating for height - the standard height for measuring wind speed is 10m. Very few amateurs can manage to get their anemometer that high, so you can apply the formula to get your 30m equivalent speed:

The formula for correcting wind speed from height h to height 10 metres is:

Code: Select all

V10/Vh = 1/(0.233 + 0.656*log10(h+4.75))
where h is in metres, Vh is the speed at height h, and V10 is the speed at 10 metres.

So, if your anemometer is 5m off the ground, V10/Vh comes out at 1.134, and that's the value that you should use as a multiplier. All other things being equal...

But of course, typically they're not, because of trees and buildings, and the landscape. I'm fortunate (in this case) in that there are no trees here, it's relatively flat, and I can site my anemometer far enough away from the house that it has little effect. I think the only way you can really get a good figure in this case is to compare your readings with a known good source nearby.
Steve
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mikentl
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Re: wind speed correction

Post by mikentl »

go to this address
http://www.zetnet.co.uk/sigs/weather/Met_Codes/beaufort.htm
Then see what the wind is doing in your area and compare it to the speeds given ie are Leaves and small twigs in constant motion then wind speed is about 8 - 12mph
then multiply your detected wind speed by a number that come out close to what the wind is doing. I found it easier during a quite blowy day

hope this helps

Regards Mike
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daj
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Re: wind speed correction

Post by daj »

Thanks both -- I think they (including Jason) are predicting high winds tomorrow up my way so can maybe use that as a starting point. It's currently calm with no air movement; the calm before the storm springs to mind!
David
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daj
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Re: wind speed correction

Post by daj »

I've had an idea. I just realised I have a spare weather station! I might go out into a field nearby and get a reading on the wind and compare it to my home station. I might look a bit of a prat, but it will not be the first time :lol:
David
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daj
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Re: wind speed correction

Post by daj »

steve wrote:
The formula for correcting wind speed from height h to height 10 metres is:

Code: Select all

V10/Vh = 1/(0.233 + 0.656*log10(h+4.75))
where h is in metres, Vh is the speed at height h, and V10 is the speed at 10 metres.
Steve, I believe the multiplier compensation figure returned here would be in m/s. If I am measuring my wind speed in mph do I need to adjust my multiplier (e.g. form m/s to mph) or does Cumulus do that for me.

Also, I assume I would set my Wind Speed and Wind Gust multipliers to be the same?

thanks
David
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Re: wind speed correction

Post by beteljuice »

I believe the multiplier compensation ....
It's a ratio ;)
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daj
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Re: wind speed correction

Post by daj »

beteljuice wrote:
I believe the multiplier compensation ....
It's a ratio ;)
I did wonder :? -- after I had submitted it I was thinking it through a little more but thought it best to leave and check.

So I assume I put the same ratio/multiplier in Wind Speed and Wind Gust calibration?
David
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steve
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Re: wind speed correction

Post by steve »

daj wrote:Steve, I believe the multiplier compensation figure returned here would be in m/s. If I am measuring my wind speed in mph do I need to adjust my multiplier (e.g. form m/s to mph) or does Cumulus do that for me.
It's a ratio, so it doesn't matter what units are in use, it's the same for all units.
Also, I assume I would set my Wind Speed and Wind Gust multipliers to be the same?
Ah, well, good question. I am told that the Met Office say that you should report your average wind speed using the height adjustment, but gusts should be reported as read. I can't actually find this in my Observer's Handbook. It is a very old copy, however.
Steve
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steve
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Re: wind speed correction

Post by steve »

steve wrote:V10/Vh comes out at 1.134, and that's the value that you should use as a multiplier.
By the way, it's been pointed out to me that (by design) Cumulus only allows 2 decimal places for the calibration values, so my statement above wasn't quite accurate :)
Steve
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daj
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Re: wind speed correction

Post by daj »

thanks, you can tell I am an amateur. :D :D But learning

I am doing a Wiki page on it and then the 'real guys' can check it for accuracy.
David
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daj
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Re: wind speed correction

Post by daj »

Can this please be checked for accuracy

Wiki -- Wind measurement

I have written it based on information gained on the forum and on the internet in general. Please amend as necessary.

I would also like to have articles on the Wiki for Rain, temperature and pressure measurement if anyone feels like picking up the gauntlet?!
David
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Re: wind speed correction

Post by skyewright »

steve wrote:Ah, well, good question. I am told that the Met Office say that you should report your average wind speed using the height adjustment, but gusts should be reported as read. I can't actually find this in my Observer's Handbook. It is a very old copy, however.
At this page I found "At stations where the effective height differs substantially from the actual height, corrections are applied to the 10-minute wind speed reported in the SYNOP message. No corrections are applied to any gusts measurements or to any hourly mean wind speeds.
Is that maybe what you were thinking of?
Regards
David

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