Low measured wind speeds in a wooded urban area

[nking]

But even here, in the middle of an acre of fairly flat land, with no trees, and no buildings nearby apart from our house, I can't find an ideal spot for my weather station.

You are fortunate enough to have a Davis and an FO station and I wondered how the wind speed results compare. In particular, in your experience would you choose the FO “gust” as being more representative of wind speed? I appreciate that your site, albeit that you feel you cannot find a ideal spot, is still much more suitable than most other FO owners sites.

[steve]

You are fortunate enough to have a Davis and a FO station and I wondered how the wind speed results compare.

I did some initial comparisons when I first got the FO, and they seemed to compare reasonably well. Some later comparisons (probably at a different wind speed) were not so good. Right now the wind is from the SSE, which means the house is getting in the way a bit, but the FO is showing 12.1 mph (48-second speed) and the Davis 18 mph. The FO anemometer is a bit lower than the Davis, so will suffer from the house effect more.

I'll try to do some proper tests at some point.

[steve]

I've just compared the readings again (all figures are 10 minute averages):

Davis 21 mph
FO (using 'gust') 22 mph
FO (using 48-second speed) 16 mph

[Orion]

I've just compared the readings again (all figures are 10 minute averages):

Davis 21 mph
FO (using 'gust') 22 mph
FO (using 48-second speed) 16 mph

FO (using 'gust') do i need to set this using the station or tick a box in cumulus ?
These are my setting at the moment

[steve]

FO (using 'gust') do i need to set this using the station or tick a box in cumulus ?
These are my setting at the moment

'Using gust' is the default, as you have currently. To get it to use the FO 48-second value for the calculation, you need these settings:

Station settings_2011-01-12_12-17-54.png

[werribee_au]

I used to 'use speed for average calculation' but noticed a considerable difference comparing the speed with my local BOM stations, but since I have had it unchecked my speed and gust pretty much follow my local BOM stations here in Australia. So I have retained that setting. Just mt 2 bobs worth

[Gina]

So it looks like the F.O. "speed" value is rather useless - it does always seem to follow the "gust" value but a bit lower. Maybe the F.O. does take the highest "gust" in 48secs , add it to the average and divide by 2 as Steve posted in the F.O. sub-forum.

24hrs-Wind-1.png

[nking]

I've just compared the readings again (all figures are 10 minute averages):

Davis 21 mph
FO (using 'gust') 22 mph
FO (using 48-second speed) 16 mph

Many thanks Steve, I think this re-enforces (for me anyway) using the “Gust” speed i.e. using Cumulus to “calculate 10 min average speed” setting.

[steve]

I'll take some more readings when the wind swings around to the north so it's not affected by the house, and also try to allow for the fact that the FO anemometer is lower than the Davis (but there's not a great deal of difference, maybe a metre).

[brassing]

I'll take some more readings when the wind swings around to the north so it's not affected by the house, and also try to allow for the fact that the FO anemometer is lower than the Davis (but there's not a great deal of difference, maybe a metre).

That would be interesting. I notice in a an earlier post, about a previous comparison, you said the FO average speed was very close to the Davis.

Even when anemometers agree in steady winds there can be reasons for them to differ in variable winds. I saw somewhere that cup anemometers react faster to increases in wind than to decreases - ie they speed up faster than they slow down. That would cause them to over read in gusty winds. The FO has a very light rotor with a low moment of inertia so might be less susceptible to this over reading than the presumably more robust rotor of the Davis (I've never seen one). i.e. maybe the FO is more accurate!

On the subject of which speed to use. Gust and average are measuring different things. I can't see the logic of using the gust figure for the average speed just because some data suggests it gives a speed value closer to the the one we want. If the average speed is too low for some reason (and this is not yet confirmed - a calibration I did (in my car) showed the FO average speed was pretty accurate at a steady 10 mph) then it would be more logical to apply a correction factor to the average speed rather than use the gust speed.

The only argument for using the gust speed IMO is that, in clear air, gusts correspond to packets of wind from higher up in the atmosphere being brought down the ground by convection and eddies. Thus the gusts possibly tell you something about the wind at higher levels. (This does not apply to gusts caused by building and tree induced turbulence.)

[steve]

Some more figures now that the wind is due south so not affected by the house.

Davis 12 mph
FO (using 'gust') 13 mph
FO (using 48-second speed) 9 mph

[nking]

Well the "gust" is still closer to your Davis. I often compare mine to othe stations in my area using this link http://www.wunderground.com/cgi-bin/fin ... ry=bn6+9rw you need to put in your own post code.... if one is looking for reassurance

[brassing]

Some more figures now that the wind is due south so not affected by the house.

Davis 12 mph
FO (using 'gust') 13 mph
FO (using 48-second speed) 9 mph

Steve - have you corrected for the height difference in those figures? How big is that correction anyway? Gina has said earlier in this topic that even 1 m can make a significant difference (at least for her setup).

[wd40]

Who was it that said a man with two thermometers does not know what the temperature is? Maybe I read it here.

Randy

[steve]

Correcting those last speeds to 10m equivalent and rounding, using this table http://wiki.sandaysoft.com/a/Wind_measurement gives:

Davis 14 mph
FO (using 'gust') 15 mph
FO (using 48-second speed) 11 mph