Wind Run Anomaly

[SouthUistWX]

I hope that people do not think that I am going over old ground, but there is something about “wind run” and Cumulus that I just can’t seem to get my head round.

I fully understand and comprehend the concept and meaning of wind run and how Cumulus calculates the value which I quote from CumulusWiki is

“by noting the average wind speed every minute, and adding in a minute's worth of 'distance' corresponding to that speed. So the wind run for a particular period is effectively an indication of the average wind speed over that period. A wind run of 240 miles over the course of a day, for example, means that the average (of up to 1,440 measurements of) wind speed over the day was 10 mph.”

If this be the case, why is there no simple correlation between our daily wind run and average speeds? For example, the readings for the last 4 days are as follows:-

DateHigh..........Gust (mph).....Average Speed (mph).........Wind Run (miles)
12/11/13............42.....................20.2............................441.4
13/11/13............53.....................19.8............................585.2
14/11/13............44.....................21.2............................356.1
15/11/13............33.....................11.5............................310.7

During this period there was no downtime and our logging interval is 10 minutes, which I believe to be within the recommended time period.

We have been running Cumulus for nearly 3 years now and it is something I have tried to understand and is now just bugging me! Is it me or is there something that I am missing.

We have a Davis VP2 and are currently running on Cumulus (1.9.3.- b1057) but this has happened on all Cumulus versions that we have run.

Please help before I go mad!

[steve]

Where are you getting those average speed figures from?

[SouthUistWX]

I get the values by entering "VIEW" - then "THIS PERIOD" when the form "Averages & Extremes for Selected Period" appears.

I then enter the start date & end date. In these cases as I wish to look at just 1 day, they are the same. The values for the Average speed, Highest gust and windrun are then populated.

Something else I think you should note is that oue weather day starts and finishes at 0900GMT.

[steve]

The "This Period" display works on calendar dates.

[SouthUistWX]

If that is the case, why, when our day file is created at 0900GMT and uploaded to our web site does it contain the same values?

Another point is that would not the wind run etc still be calculated over a 24 hour period and therefore still have the same correlation, ie windrun = av wind speed x 24?

[steve]

If that is the case, why, when our day file is created at 0900GMT and uploaded to our web site does it contain the same values?

Sorry, I don't understand. Dayfile.txt doesn't contain the average wind speed.

Another point is that would not the wind run etc be calculated over a 24 hour period and therefore still have the same correlation, ie windrun = av wind speed x 24?

Why would it be the same, when the wind run (in dayfile.txt) is calculated using data from 0900-0900 (in your case) and the average speed on the 'this period' display is using data from midnight to midnight?

[SouthUistWX]

Sorry Steve, yes, it has the same values for high gust and wind run and average speed is not uploaded.

My point, and to my mind is that firstly, if the dayfile is calculating values from 0900 - 0900, why would it contain the same values as another which calculates it from midnight to midnight?

My second point is , that if "this period" is calculating values between 0000 - 0000, I am not saying that they should be the same, but would there not still be the correlation between average speed and windrun that I expect?

Sorry if i am coming across as Mr Angry. I'm not really at all

[steve]

Sorry Steve, yes, it has the same values for high gust and wind run and average speed is not uploaded.

My point, and to my mind is that firstly, if the dayfile is calculating values from 0900 - 0900, why would it contain the same values as another which calculates it from midnight to midnight?

Sorry, I wasn't precise in my reply, as we were only discussing average wind speed. The "This Period" display of average wind speed works on calendar dates. The NOOA report, as I noted in a recent thread, is the same (because I lifted the code). Hopefully at some point I'll get chance to correct them both.

My second point is , that if "this period" is calculating values between 0000 - 0000, I am not saying that they should be the same, but would there not still be the correlation between average speed and windrun that I expect?

No, not necessarily. There are nine hours of different data involved, i.e. well over a third.

You can calculate your approximate 0900-0900 daily average wind speed from the figures logged in the monthly log file by finding the average of all of the average wind speed figures (the sixth column) for an 0900-0900 period (which is how the 'this period' display does it, but it uses midnight to midnight because it only uses the calendar date), e.g. by loading a copy of the log file into Excel. I did that for my figures for the 16th November (I also use 0900-0900) and my average is 25.2 mph. My wind run for that 0900-0900 day is 603.9 miles, which is equivalent to an average speed of 25.2 mph.

[SouthUistWX]

OK Steve, Thanks very much for that and hope I haven't wasted your time. I think I've grasped the situation and glad that I'm not going mad and will delve into data and work it out if only for my peace of mind.

Thanks very much once again.