[wxqc] Average wind direction?

[Victor Engel]

On 8/10/06, steveh at softwx.com <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.
>

I agree. For that purpose, the difference wouldn't be very significant. By
the way, in trying out various scenarios, I discovered something I was
previously unaware of regarding local weather. It seems to be dependent on
time of day. Over the past couple months, morning winds have averaged
southerly and evening winds have averaged easterly. Now I'm curious if this
is a general trend here. Do you know of any sites that compile this sort of
statistics? I first noticed this pattern when I graphed hourly direction
over the period I have data for. It made a very clear sinusoidal pattern.

I have another observation, perhaps more on-topic. I've noticed that I have
zero data points with wind direction between 0 and 5 or between 5 and 9
degrees. Same for between 355 and 360 degrees. For directions further away
from 0, all directions are present. I assume this is somehow related to the
design of the wind direction sensor, but I'm curious about the nature of the
imprecision. Or maybe the sensor senses only multiples of 22.5 degrees. Does
anyone have any insight into this?

Here are histograms for Weatherlink, Weatherdisplay, and Wunderground. They
cover different time periods, so ignore the overall trend. Instead, pay
attention to the detail as the x-value changes slightly. Are these
non-(22.5degree multiples) a result of the various software packages
doing averaging?
Does the absence of 1..4, etc. indicate an error in the averaging algorithm?
http://the-light.com/weather/weatherlink.pdf
http://the-light.com/weather/weatherdisplay.pdf
http://the-light.com/weather/wunderground.pdf
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