Author Topic: Anemometer height above roof line  (Read 730 times)

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Offline SCP374

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Anemometer height above roof line
« on: October 31, 2017, 01:38:19 PM »
I know there have been past posts about this, but I am curious to know how much of a difference my wind readings will be if I add a 31" extension to my current wind mast.  I'm using the weather mount from ambient weather. The pole height is approximately 7' above the gable now.  Does anyone know if the extension will make a big difference in readings?  I do realize that roofs cause some turbulence as well.  My roof has a fairly steep pitch as it's a cape cod style home.  Pic attached to show the current pole.  Any feedback would be most appreciated!

Offline dalecoy

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Re: Anemometer height above roof line
« Reply #1 on: October 31, 2017, 01:50:36 PM »
I know there have been past posts about this, but I am curious to know how much of a difference my wind readings will be if I add a 31" extension to my current wind mast.  I'm using the weather mount from ambient weather. The pole height is approximately 7' above the gable now.  Does anyone know if the extension will make a big difference in readings? 

Define "big difference".

Roofs will have an effect on wind measurements, up to at least 20 feet above the roof.  Adding 31" will make some difference.

Offline SCP374

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Re: Anemometer height above roof line
« Reply #2 on: October 31, 2017, 02:09:48 PM »
I shouldn't say "big difference" but I meant - would a 31" height increase lead to higher readings, even just a few mph?  I have full exposure on the SW and W sides which is the prevailing direction up here in Massachusetts.  There is a tidal river across the street and no trees blocking the wind for about 1/2 mile in that direction.  It seems that the wind gusts should be higher than what I actually record on both instruments, so I was wondering if the roof was causing turbulence and reducing the readings?  I also realize that the pole would wobble or sway more in higher wind speeds if it was extended in height.  Would that affect readings?

Offline dalecoy

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Re: Anemometer height above roof line
« Reply #3 on: October 31, 2017, 05:24:26 PM »
A 31" extension would result in changed measurements.  Probably, most of those changed measurements would be more accurate.  Depending on wind direction, and a large number of other factors.

Yes, my answer is vague and weasel-worded.

Offline SCP374

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Re: Anemometer height above roof line
« Reply #4 on: November 01, 2017, 09:17:35 PM »
Thanks for your feedback.  So even 2 1/2' will make a noticeable difference?  Does anyone know how far away a tree has to be to have an effect on wind speed?

Offline CW2274

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Re: Anemometer height above roof line
« Reply #5 on: November 01, 2017, 09:32:22 PM »
Does anyone know how far away a tree has to be to have an effect on wind speed?
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Online hacker

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Re: Anemometer height above roof line
« Reply #6 on: January 08, 2018, 11:31:24 AM »
My Vue consistently reads lower wind speed than the TV reports, and as an experienced motorcycle rider I can tell the difference between 15mph and 40mph wind. Problem seems to be turbulence caused by buildings, trees and terrain. First mounting was 4 ft above roof peak of barn. Moved it to a 21ft pole away from barn and the readings increased, but we are on a steep hill 200ft above a lake with tall pines about 100ft from ISS. I suspect it would require a much taller pole to escape the turbulence and get accurate wins speed.

Offline LABob

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Re: Anemometer height above roof line
« Reply #7 on: January 08, 2018, 11:42:20 AM »
What does "accurate" even mean in this context? Are you trying to measure how fast the wind is blowing on the ground where people and buildings feel it, or way up where only birds and bugs feel it?

Offline Old Tele man

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Re: Anemometer height above roof line
« Reply #8 on: January 08, 2018, 01:04:05 PM »
The desired "30-feet" anemometer height is to get above the surface-wind "ground effects" due to buildings, trees, etc.. There are equations available for describing the translation of winds from "clear air" (above 30 ft) down to "dirty" (5-6 ft) surface winds.
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Offline openvista

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Re: Anemometer height above roof line
« Reply #9 on: January 08, 2018, 01:21:40 PM »
Here's a link that helps you extrapolate wind speeds to a certain height: https://websites.pmc.ucsc.edu/~jnoble/wind/extrap/. Keep in mind that 10m is the ideal height for an open field with no obstructions. In my case, the developed environment (small town with buildings close together) means I would need to place my anemometer at least 70ft high to capture forecasted wind speeds. YMMV. Instead, I put my anny at the top of a 30' pole and exposed it to prevailing winds and called it a day.

Tim Oke, writing for the WMO, talks at length about siting for urban wind measurement. See attachment.
« Last Edit: January 08, 2018, 01:24:23 PM by openvista »
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Offline dalecoy

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Re: Anemometer height above roof line
« Reply #10 on: January 08, 2018, 01:31:47 PM »
My Vue consistently reads lower wind speed than the TV reports, and as an experienced motorcycle rider I can tell the difference between 15mph and 40mph wind. Problem seems to be turbulence caused by buildings, trees and terrain. First mounting was 4 ft above roof peak of barn. Moved it to a 21ft pole away from barn and the readings increased, but we are on a steep hill 200ft above a lake with tall pines about 100ft from ISS. I suspect it would require a much taller pole to escape the turbulence and get accurate wins speed.

Good information provided by others - especially by openvista. 

Remember that, with a Vue, there is no way you would be able to duplicate "TV reports" for wind and for temperature/humidity.

But the key question is:  where do you want to have measurements of the various parameters?  Is it important to you, to know the wind/temperature/humidity at the top of a tall pole?  Or above your barn?  Or where you live?

Offline Old Tele man

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Re: Anemometer height above roof line
« Reply #11 on: January 08, 2018, 01:46:43 PM »
Here's a link that helps you extrapolate wind speeds to a certain height: https://websites.pmc.ucsc.edu/~jnoble/wind/extrap/. Keep in mind that 10m is the ideal height for an open field with no obstructions. In my case, the developed environment (small town with buildings close together) means I would need to place my anemometer at least 70ft high to capture forecasted wind speeds. YMMV. Instead, I put my anny at the top of a 30' pole and exposed it to prevailing winds and called it a day.

Tim Oke, writing for the WMO, talks at length about siting for urban wind measurement. See attachment.
Figure 4 on page 22 summarizes everything visually...

Previous folder on this subject: http://www.wxforum.net/index.php?topic=2753.msg23379#msg23379
« Last Edit: January 08, 2018, 01:52:45 PM by Old Tele man »
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Re: Anemometer height above roof line
« Reply #12 on: January 08, 2018, 08:08:06 PM »
Looking at the correction tables found by following Old Tele man's link, I should multiply my reading by about 1.4x for a 3m ISS height. That sounds pretty close to my guesstimates.

 

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