Problem with wind gust value

[broadstairs]

I was comparing the max recorded wind gust today from my VP between Weather Display and Cumulus and there is a significant discrepancy, and remember since they both get the data from VirtualVP it should be the same. Cumulus recorded a max gust of 37mph at 14:19 and Weather Display recorded 44.8mph at 14:19, now WD has a +10% factor to take into account because all home anemometers under read by at least 10% and probably more in a residential area. So on that basis I would have expected WD to show 40mph if the Cumulus value was correct, or Cumulus should have read 40.7mph if WD +10% is correct. I need to get to the bottom of this....

Stuart

[steve]

What does your VP2 console say?

Steve

[broadstairs]

Steve I'd forgotten that this was displayable as I never use the console, it is a VP1 by the way. Well it shows 37mph which means Cumulus is correct. Now I need to get to the bottom of why WD is reading that high. Thanks for reminding me about the console.

Stuart

[steve]

Are you sure WD doesn't add 20%, because that's close to the figures you've quoted. Is the amount configurable, otherwise it seems a bit odd, adding an arbitrary figure like that? As an aside, displaying 44.8 mph is spuriously accurate as the VP wind speed resolution is only 1 mph.

Steve

[broadstairs]

WD has two values it increases by (I'd not looked at this for some considerable time), it adds a value to the wind speed and then adds another value to gust speeds, so if you set both to 10% you get 10% added to wind speeds and 21% (in total ie another 10% to the new value) to the gust speeds. Instinctively I feel this is wrong but I'm trying to find out the logic behind what is happening. You also have to understand that WD always internally works in Knots (Brian the author likes Knots) and then he converts everything which is why you end up with values which include decimal points which the VP does not give.

In my view being able to add a % to wind speed is good since most domestic anemometers read under by between 10 & 20% because they are not installed in ideal circumstances, however this single adjustment should apply at all speeds. That's my view anyway.

If I'm proved wrong (perfectly possible) then I'll post again on this thread....

Stuart

[steve]

Ah, that makes sense. Well, apart from the bit about the gust getting both lots of 10%, I can't get my head around that bit at all

[steve]

Regarding spurious accuracy, I've just noticed that Cumulus displays mph wind speeds to 1 dp on VP and VP2 stations. I don't know how or when this happened, it used to display whole numbers. I guess I should fix that...

Steve

[broadstairs]

Steve there has been some discussion on the WD forum but as yet no reply from the author. There is some suggestion that inertia could be higher for gusts over more constant wind speeds and that may be why the additional increase is added however how you would try to address this is not clear and I'm not convinced either. I have removed this second adjustment from my WD installation but left the single +10%.

I do think that it would be good for at least a single adjustment factor for wind speed in Cumulus would be good because as I have said most home anemometers do under read by probably 10% or perhaps a bit more simply because they are usually not in ideal locations. Again an adjustment factor for direction could be provided (+ or - a number of degrees) to allow fine tuning for direction as exactly lining up the wind direction hanging on to the top of a ladder when you put it up is not the easiest. However that is as far as I'd go for wind. Rain is another area where hom gauges are prone to under read so an adjustment for rain might be useful so people can use software to calibrate their gauges.

Stuart

[dc1500]

Stuart, I may set up a little project looking at rain gauges. I have one Oregon type that is round and tips every 1mm. I also have the smaller oblong type (Lacrosse, Watson etc) which tips every 0.3mm. This seems to give higher readings. I previously calibrated the former against a standard (fairly) rain gauge and results were good for higher falls. I can do this quite accurately because I weigh the water which is a very accurate way of getting the volume. It seems logical that if you get a low amount of rainfall, say 1.8mm the former will read 1mm the latter 1.8mm and if it doesn't rain for some time this 0.8 mm could evaporate from the bucket of the 1mm tipper. Also traces could do this without accumulating. I'll report the results in a month or so (We don't get much rain in S.Essex, e.g 27mm this September!)

[broadstairs]

Steve the WD author (Brian known as Windy on the WD forum) has replied to my question and it appears that there is a logic error in WD.

The situation is that there are some types of weather station which provide two wind speed readings the normal average and the gust and for these stations you have two adjustment factors one for each and each is only applied once.

However, and this is where the logic error came in, Davis VP stations (1 & 2) only have a single wind speed sent (about every 2.5 seconds) so the average has to be computed and the highest in the period recorded as the gust, so in this case both adjustment factors were getting added to the readings and hence my problem. So you need to watch out for this when handling wind speed offsets or calibration values for Davis VP systems as you only need a single value.

Stuart

[steve]

The VP and VP2 send two wind speeds. One is the 'current' speed, which as I understand it is the speed over the previous 2.5 seconds - effectively the gust speed, and the second speed is the 10-minute average speed.

From what you say, presumably WD ignores the 10-minute average supplied by the station and calculates its own average from the first of the two values. Cumulus gives the option of doing it that way or by simply using the 10-minute average as supplied. This is the difference between having the 'calculate 10 minute average' option set or unset.

In the case where you are calculating the average from the gust/current speed, I can see how a programming error could arise where the 10% gust adjustments were applied to the gusts and then those adjusted figures were used to calculate the average speed, which then itself had the 10% wind adjustments applied to it.

I'm amazed that you're first WD user to notice this!

What Cumulus does is to only apply the wind speed multiplier if the station-supplied average is being used. (So in the Cumulus-calculated option, the gust multiplier also governs the average speed and the wind multiplier is ignored).

Steve

[broadstairs]

Steve there was a change at some time in the past but after I initially setup the adjustment. When I did this setup there was only one value to setup.

Yes I gather that WD does not use the VPs 10 minute average but does its own. I was only quoting what Brian said about the VP wind speeds.

I'm glad you clarified how Cumulus works as I'd have applied the wrong adjustment factor. I have asked Brian to clarify which value should be used in WD as well since his logic may not be your logic!

Stuart

[steve]

I've been thinking about the adjustments and I think I will change the way I have currently implemented them. A bit of background: Cumulus reads the two values from the station, the 'current' and the 'average'. It tracks the highest 'current' in the last 10 minutes and reports this as the 'gust'. Depending on the option selected, it either uses the supplied average, or calculates a 10-minute average based on the set of 'current' values, and displays this as 'average'. It is these 'gust' and 'average' figures that get plotted on the graphs.

I propose that the 'gust' adjustment should just affect the calculated 'gust' figure (i.e. peak wind in last 10 minutes), and the 'speed' adjustment should just affect the 10-minute average, whether Cumulus calculates it or not. The gust and speed adjustments are never applied simultaneously to the same data. I think this probably achieves what most people would expect when setting 'gust and 'speed' adjustments, and better separates the use of the two adjustments?

Steve