[wxqc] Average wind direction?

[Victor Engel]

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 <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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