[wxqc] Average wind direction?
Mark Wyman
mark at markwyman.com
Fri Aug 11 11:41:21 EDT 2006
This whole subject is quite interesting, as I have been doing research on my
hill for installing a wind turbine. I want to find the wind energy over a
period of a year, and prevailing direction. The example I posted before will
find this for me. All of the samples I have so far are in 2-minute average
periods, not gusts.
Now on another subject: my concept of a wind rose. I am no mathematician but
I was hoping to calculate a "curve fit" wind rose that would draw an ellipse
based on average values over time, and then draw these ellipses based on
2-minute, 10-minute, hour, day, etc in differing colors overlaid on the same
graph.
If over a period of time the wind was only one angle, the ellipse would be
almost a straight line (flat) pointing in the direction of the wind. One end
would be pivot point. If the wind was random, it would be circular and the
pivot would be in the center. I am sure this is possible to do, and then I
want to make a control out of it that can be inserted in any program and
re-used.
To me it would help visualize wind energy, and then the graph could be
tilted in space to show history (3d) of these readings.
If only I had a bit more time on my hands.
-Mark
-----Original Message-----
From: wxqc-bounces at lists.gladstonefamily.net
[mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of
steveh at softwx.com
Sent: Friday, August 11, 2006 2:24 AM
To: Discussion of weather data quali
Subject: Re: [wxqc] Average wind direction?
Hi,
Although helpful, I found in those links the same problem I encountered
before. They typically just state something like "2 minute average", but
they don't specify whether wind speeds are scalar or vector averages, and
don't state whether wind directions are unit vector averages or vector
averages that include speed. I was left to having to make an assumption, and
chose scalar average wind speeds and unit vector wind directions. I don't
have a preference; one way is as easy to code as another, but I was hoping
to find a definitive statement of what is expected.
I know to some this may seem off topic, but if the processes ingesting data
are assuming observations using one standard, and the software submitting
the data is using a different standard, I would think that has an effect on
data quality.
Steve
======= At 2006-08-10, 21:43:22 you wrote: =======
>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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