[wxqc] Average wind direction?

[Dave Helms]

FYI...

Federal Surface Obs Spec (FMH-1)
http://www.ofcm.gov/fmh-1/fmh1.htm
http://www.ofcm.gov/fmh-1/pdf/E-CH5.pdf
"5.4.1 Wind Direction. The wind direction shall be determined by 
averaging the direction over a 2-minute period."

ASOS Raw Data Processing to meet FMH-1: 
http://www.nws.noaa.gov/com/nwsfocus/fs20050803.htm#new
http://www.nws.noaa.gov/ops2/Surface/documents/IFW_PNS_TEMPLE-2.pdf
http://205.156.54.206/asos/
http://205.156.54.206/asos/aum-toc.pdf
Note:  ASOS sonic anemometer (e.g. Ice Free Winds) replacement of rotory 
cups in 2005

More FAQs:
http://www.fs.fed.us/raws/photos/windstudy/windstudy.shtml
http://www.nws.noaa.gov/ops2/Surface/documents/IFW_PNS_TEMPLE-2.pdf
http://www.aopa.org/asf/publications/sa09.pdf

The CWOP Guide recommends a 2 minute average wind speed and direction, 
and a 10 minute peak gust, based on the FMH-1 and WMO specifications.*


*

steveh at softwx.com wrote:

> I don't know (but would like to know) what the "official" NWS standard 
> is for calculating wind direction observations. When I was writing 
> VPLive, I hunted around and found references for both weighted vector 
> avg and unit vector avg. Here are some examples for unit vector avg:
> http://www.ndbc.noaa.gov/wndav.shtml
> http://www.glerl.noaa.gov/metdata/chi/archive/
>  
> The diffence between the unit vector avg formula I gave vs. the 
> weighted vector avg is not a big deal. I'd be happy to implement it 
> whichever way jives with the NWS preference.
>  
> As for which is better, I don't know. Similar to the example you gave, 
> what if a north wind blew 20 mph for an hour  and then blew from the 
> east at 1 mph for the next 9 hours. Would you say the wind over the 10 
> hours was from the NNE or ENE? Practically though, since the period of 
> time we're talking about for CWOP is only 2 minutes, I don't think 
> there will be much difference between either of the methods.
>  
> Steve
> ------------------------------------------------------------------------
> Why wouldn't you weight it by speed? Suppose you had a northerly wind 
> at 20 mph all morning and an easterly wind at 1 mph all afternoon. 
> Would you really want the average wind direction to be NE?
>
> Another idea would be to simply add vectors as if you were following a 
> particle of air around. After the time over which you want to average, 
> the average direction would simply be the direction of the particle. I 
> guess this is essentially the same as my original proposal but simply 
> summing instead of averaging. The only difference between the sum and 
> the average is the scalar portion of the resultant vector, which you 
> don't care about for direction.
>
> On 8/10/06, *steveh at softwx.com <mailto:steveh at softwx.com>* 
> <steveh at softwx.com <mailto:steveh at softwx.com>> wrote:
>
>     Hi,
>
>     For wind direction, you could use a unit vector average (not
>     weighted by speed).
>     Steps:
>     1. Convert wind dir degrees to radians
>     2. x = sin(winddir in radians)
>     3. y = cos(winddir in radians)
>     4. Vx = avg(x)      //i.e. sum(x) / number of samples
>     5. Vy = avg(y)
>     6. avg wind dir in radians = arcTan(Vx / Vy)
>     7. convert avg wind dir to degrees
>
>     Note that you have to include code to fixup the resultant
>     quadrant. In Object Pascal (Delphi) there is an function that
>     simplifies this; in step 6 you would use
>     avgWindDirRad := ArcTan2(Vx, Vy);
>
>     and after converting to degrees...
>     if (avgWindDirDeg < 0) then avgWindDirDeg := avgWindDirDeg + 180;
>
>     Here is a source for some detailed info on the topic:
>     http://www.bom.gov.au/bmrc/csr/prc/usepa-met-obs,4.5.99.rtf
>
>     Steve
>
>     ======= At 2006-08-10, 10:46:20 you wrote: =======
>
>     >So, as I'm slowly making progress on this Perl script in the bits of
>     >time I get here and there, I'm stumped on how to come up with a wind
>     >direction average.  Maybe I'm putting too much thought into this?  I
>     >have several documents that describe the data fields, but no one
>     >document is clear on all of the elements (i.e., one document says
>     the
>     >avg wind speed is an average over 5 minutes, another one says it's a
>     >10 minute sustained (minimum?) wind).
>     >
>     >Anyway I Google'd for info on average the wind direction and appears
>     >some of them try to take into account hte wind speed as well as part
>     >of that calculation.  I assumed it was just a an average sample based
>     >on direction alone.  Am I overanalyzing this?
>     >
>     >What formulas are people using to calculate the average wind
>     direction
>     >in their own programming?
>     >
>     >--
>     >Joe Morris
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