How much variation in wind direction is normal?

[martinu]

I've mounted my Davis Vantage Vue remote sensor on a pole pointing south, as in the instructions. It's about 15 feet above the ground and about 3 feet above the level of a hedge that is about 30 feet away horizontally. A two-storey house is about 60 feet away. Otherwise there are no obstructions.

It's a fairly calm day: average 5 mph with gusts up to 10 mph.

My wind direction results show a very large about of scatter. See http://imageshack.us/a/img59/1713/fn72.jpg. Ignore everything before 13:00 which is when I mounted the sensor on the pole. Also ignore the blip around 1600-1630: the wind vane fell off because I evidently hadn't tightened it properly after temporarily removing the vane and cups while I fitted the pole.

Website is http://www.martinunderwood.f9.co.uk/Weather/

[steve]

That looks like the kind of output you would normally get with a Fine Offset station. I don't think I've ever seen a Davis looking quite that 'random'. It seems even more strange because, by the sound of it, you have very good exposure. And your current direction is NW-ish and it should be SW-ish. I would say you have some kind of problem, but I don't know what it is, I'm afraid. Is the vane fastened on firmly?

[martinu]

That looks like the kind of output you would normally get with a Fine Offset station. I don't think I've ever seen a Davis looking quite that 'random'. It seems even more strange because, by the sound of it, you have very good exposure. And your current direction is NW-ish and it should be SW-ish. I would say you have some kind of problem, but I don't know what it is, I'm afraid. Is the vane fastened on firmly?

The SE/NW issue is user error: I'd compared an online weather forecast which showed a wind direction arrow with the head pointing downwind, with the arrow head on the wind direction gauge in Cumulus which points upwind and seen that they were they same and so assumed that everything was OK, not realising that they should be 180 degrees apart and that I'd (somehow!) got a 180 degree correction on the Calibration screen. My wife soon spotted that and got me to investigate and correct it! As an aside, I wonder if the wind direction gauge in Cumulus should have the arrow head pointing towards the centre rather than away from it to show that if it is at (for example) 180 degrees / South, that means that the wind is blowing *from* the south, not in the direction of the arrow head as it is currently shown.

The scatter is weird. I'll check again tomorrow (when it's daylight) that the vane really is screwed firmly on and calibrated according to Davis's instructions.

Even after correcting for the upwind/downwind thing, there's still a 90 degree shift: the wind is predominantly from the SW but Cumulus (and the Davis console) are reporting winds predominantly from the SE. Time to point the vane to the south and see what direction is reported!

[martinu]

That looks like the kind of output you would normally get with a Fine Offset station. I don't think I've ever seen a Davis looking quite that 'random'. It seems even more strange because, by the sound of it, you have very good exposure. And your current direction is NW-ish and it should be SW-ish. I would say you have some kind of problem, but I don't know what it is, I'm afraid. Is the vane fastened on firmly?

I've now checked that the vane is securely screwed to the shaft and is correctly aligned.

If I point the arrow head north (and sellotape it to the pole to give me time to go back inside to look at the computer) the Davis console and Cumulus both read north. If I point it south, it reads south. I've lined up the solar cell (which is said to be south) with where the compass says south is and where the OS map shows it to be.

But still I get a lot of randomness in the readings and a systematic error of about 90 degrees in the average (ie it reads NW when the prevailing wind at the moment is SW). And I can see the vane is blowing all over the place even though the air is still (3-5 mph on the anemometer and tree branches are barely twitching) - as I watched just now I saw the vane go rapidly from NW (where it had been static for several seconds) to SE (where it again was static for several seconds), and I even saw it make two complete revolutions!

Overnight the average shifted over the space of about 2 hours from 150 to 300 degrees: http://img203.imageshack.us/img203/1416/afyj.png

I must be doing something very silly with the placement of the sensor for it to be picking up huge random fluctuations in direction. But what...

[steve]

I don't have first-hand experience of the Vue. The only things I can think of are:

(a) It's not level
(b) The vane itself is broken/has a piece missing (e.g. it should have some kind of weight attached)
(c) You have some strange 'eddies' going on

I think you probably need to speak to Davis and/or your supplier.

[martinu]

I don't have first-hand experience of the Vue. The only things I can think of are:

(a) It's not level
(b) The vane itself is broken/has a piece missing (e.g. it should have some kind of weight attached)
(c) You have some strange 'eddies' going on

I think you probably need to speak to Davis and/or your supplier.

Thanks for your suggestions.

I hope I've got the pole reasonably vertical (concrete post that it's attached to wasn't but I hope I've compensated for that).

The vane looks complete - it's brand new.

I'm about to email Davis to ask their advice.

I did wonder about strange eddies in the garden - hopefully Davis can advise on things to avoid when choosing a location.