Siting a Davis Vantage Vue to give sensible wind direction

[martinu]

I've mounted my Vantage Vue on a pole (the one that Davis sell) which is fastened to an old concrete washing-line post. It's about 10 feet above the ground with clear access and mainly flat ground on two sides (NE and SE) for about half a mile. On the NW there is a beech hedge about 8 feet high and about 10 feet away horizontally. On the SW there is a two-storey house, the middle of a terrace of three houses, which is about 30 feet away.

Although the concrete post is not vertical, I've compensated for this in my mountings and if I put a spirit level against the pole, the pole seems to be vertical in two directions at right angles, such that the bubble of the spirit level is within the guide marks, even if not perfectly symmetrically within them. I can't see the spirit level that Davis fit on the top of the sensor unit because I haven't got a high enough ladder to look down from above!

Now this may fall short of the ideal of a wind vane and anemometer mounted 30 feet high, at least 6 feet above the level of a roof or any other obstruction, but would it be responsible for the bizarre wind direction readings that I'm getting?

The wind direction value is very variable, even in light wind of 3-5 mph. The average value that Cumulus reports seems to remain steady for several hours but then over the space of about half an hour the average flips by 120-180 degrees and then remains steady at this value for several hours.

The average values seem to bear no relationship to the predicted direction from weather forecasting sites such as The Weather Outlook.

I've checked that I've got the Vantage Vue's solar cell pointing as close to south as I can manage, using a compass and checking this against an OS map. By temporarily taping the wind vane with its arrow head pointing towards the pole and away from the solar cell, I can see that the Davis console and Cumulus both report a direction of north; by then taping it with the arrow pointing towards the cell and the tail pointing towards the pole, I get a reading of south - exactly as I would expect. So it's not a communications problem.

The vane seems to move around a lot. I've seen several occasions where it remains fairly steady on one heading and then flips rapidly by 180 degrees and remains steady on the new heading. I've also seen cases where the vane has spun round and made two complete revolutions in a couple of seconds before going back to being fairly steady again.

Here's a wind direction graph from Cumulus:

http://imageshack.com/a/img203/1416/afyj.png (recorded overnight on 7 and 8 Jan 14)

Notice the rapid flip in direction that took place about 07:00!

Most of the time the reported average wind direction seems to be roughly from SE or NW, at a time when the TWO site predicts wind from SW.

I've emailed Davis for advice (they don't seem to have a discussion forum) but only had an automated response "we're very busy because of the holiday so it may take us longer than normal to reply".

[Tau Bootis]

Hi
That seems perfectly normal to me, are you sure it was on the morning of the 8th and not the 9th as this trend can be seen on other weather stations at 0700 on the 9th see this site for example http://www.wunderground.com/weatherstat ... &year=2014
This step change in wind direction can happen when the pressure changes as it did in this case.

[martinu]

Hi
That seems perfectly normal to me, are you sure it was on the morning of the 8th and not the 9th as this trend can be seen on other weather stations at 0700 on the 9th see this site for example http://www.wunderground.com/weatherstat ... &year=2014
This step change in wind direction can happen when the pressure changes as it did in this case.

You've got me wondering, but according to the timestamp on the file that I saved from c:\cumulus\web\images, it was definitely 8 Jan.

So you're saying that sudden shifts in wind direction and a large amount of variation about the mean value are normal?

How about these, which date from a few minutes ago (11 Jan) which show a shift from SW to E and back to SW taking place over about an hour, while wind speed was falling to almost zero and pressure was increasing steadily apart from a short period earlier in that day when it stayed steady.

http://imageshack.com/a/img854/194/g78o.png (wind dir)
http://imageshack.com/a/img18/4097/99sd.png (wind speed)
http://imageshack.com/a/img811/4513/0zrh.png (pressure)

If this is normal, then that's fine, but Steve the site admin though the amount of variation looked too random for a Vantage and would only be expected with a Fine Offset.

Should I be expecting that the wind direction that I see will be mostly similar to the forecast given on (for example) http://www.theweatheroutlook.com/ for my location (North Yorkshire, coincidentally not a million miles from the Guisborough site that you referred me to earlier).

[jdc]

There does seem to be a lot of 'scatter', particularly on the direction graphs. If the vane is moving the airflow must be causing it to turn. I would suggest that a two storey house 30 feet away to the SW (prevailing at the moment) would be quite a source of turbulence. The hedge may contribute too.

Could you use something to make some smoke and then you'd see what the airflow actually is doing?

[Tau Bootis]

So you're saying that sudden shifts in wind direction and a large amount of variation about the mean value are normal?

Well as far as I can tell from looking at your trends they appear to be ok and and far better than my Fine Offset (when its working) the latest deviation in direction was when the wind dropped and as John has just pointed out any local obstructions will cause turbulence even on a Davis, here is today's trend from a site with a Vantage near Middlesbrough notice the dip at 1430 similar to yours at 1500.

wind.jpg

To get perfect direction you need perfect placement something not many of us can achieve; I had to move mine from a similar position to yours up onto my TV aerial and even now it still suffers from turbulence.

As far as trying to compare with an official wind forecast I would take it with a pinch of salt you only have to look at the this wind map to see it is blowing in all directions at the same time.