Wind Direction - weekly/monthly/yearly

[Elton Prytz]

Hi all

Current weather conditions at my location are such that the predominant winds are from the North. In reality this means that they normally vary from NW to NE. All appears ok on the daily plots but there appears to be a problem on the weekly/monthly/yearly plots.

Here is a current weekly plot:-



If one looks at the "Wind Vector" plot for the 29th (lower graph) it is clear that the dominant direction was from the North. However the "Wind Direction" plot (upper graph) shows the wind direction as around 180deg or South. I assume that there is some averaging and the result of half the time being at 10deg and half the time at 350deg is being interpreted as (10 + 350)/2 = 180. Similar results for monthly and annually as far as I can see.

Thanks and regards
Elton








f

[Thomas Keffer]

Hi, Elton

Unfortunately, the plot is not showing up, at least in my email.

Weewx does vector averages, not simple averages of the wind direction. That is, a wind is resolved into x and a y components and those are averaged separately. The resultant two components then make up the "dominant wind direction." So, the scenario you outline would be calculated correctly as an average wind from the north.

Can you try again with your plot? It's hard to comment without seeing it.

-tk

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[Elton Prytz]

Hi Tom

Image of plot attached.

Elton

On Mon, 2013-04-01 at 13:33 -0700, Thomas Keffer wrote:

Hi, Elton

Unfortunately, the plot is not showing up, at least in my email.

Weewx does vector averages, not simple averages of the wind direction. That is, a wind is resolved into x and a y components and those are averaged separately. The resultant two components then make up the "dominant wind direction." So, the scenario you outline would be calculated correctly as an average wind from the north.

Can you try again with your plot? It's hard to comment without seeing it.

-tk

On Mon, Apr 1, 2013 at 9:46 AM, Elton Prytz <el...@promicro.co.za> wrote:

Hi all

Current weather conditions at my location are such that the predominant winds are from the North. In reality this means that they normally vary from NW to NE. All appears ok on the daily plots but there appears to be a problem on the weekly/monthly/yearly plots.

Here is a current weekly plot:-

[Thomas Keffer]

Hi, Elton

That does look odd.

One more request - could you also send the relevant section in skin.conf?

-tk

[Elton Prytz]

Hi

If you need to look at any of the other graphs then www.promicro.co.za/weather

Relevant skin.conf follows:-

   
    # Default plot and aggregation. Can get overridden at any level.
    plot_type = line
    aggregate_type = none
    width = 1
    time_length = 86400    # == 24 hours

    [[day_images]]
        x_label_format = %H:%M
        bottom_label_format = %m/%d/%y %H:%M
        time_length = 86400    # == 24 hours
       
        [[[daybarometer]]]
            [[[[barometer]]]]
       
        [[[daytempdew]]]
            [[[[outTemp]]]]
            [[[[dewpoint]]]]
       
        [[[daytempchill]]]
            [[[[windchill]]]]
            [[[[heatindex]]]]
       
        [[[dayrain]]]
            # Make sure the y-axis increment is at least 0.02 for the rain plot:
            yscale = None, None, 0.02
            plot_type = bar
            [[[[rain]]]]
                aggregate_type = sum
                aggregate_interval = 3600
                label = Rain (hourly total)
               
        [[[dayrx]]]
            [[[[rxCheckPercent]]]]
       
        [[[daypond]]]
            yscale = None, None, 0.5
            [[[[extraTemp1]]]]
       
        [[[daywind]]]
            [[[[windSpeed]]]]
            [[[[windGust]]]]
       
        [[[dayinside]]]
            [[[[inTemp]]]]
       
        [[[daywinddir]]]
            # Hardwire in the y-axis scale for wind direction:
            yscale = 0.0, 360.0, 45.0
            [[[[windDir]]]]
            line_type = none
            marker_type = circle   

        [[[daywindvec]]]
            [[[[windvec]]]]
                plot_type = vector

        [[[dayradiation]]]
            [[[[radiation]]]]
           
    [[week_images]]
        x_label_format = %d
        bottom_label_format = %m/%d/%y %H:%M
        time_length = 604800    # == 7 days
        aggregate_type = avg
        aggregate_interval = 3600
       
        [[[weekbarometer]]]
            [[[[barometer]]]]
       
        [[[weektempdew]]]
            [[[[outTemp]]]]
            [[[[dewpoint]]]]
       
        [[[weektempchill]]]
            [[[[windchill]]]]
            [[[[heatindex]]]]
       
        [[[weekrain]]]
            yscale = None, None, 0.02
            plot_type = bar
            [[[[rain]]]]
                aggregate_type = sum
                aggregate_interval = 86400
                label = Rain (daily total)
       
        [[[weekpond]]]
            yscale = None, None, 0.5
            [[[[extraTemp1]]]]
       
        [[[weekrx]]]
            [[[[rxCheckPercent]]]]
       
        [[[weekwind]]]
            [[[[windSpeed]]]]
            [[[[windGust]]]]
                aggregate_type = max
       
        [[[weekinside]]]
            [[[[inTemp]]]]
       
        [[[weekwinddir]]]
            yscale = 0.0, 360.0, 45.0
            [[[[windDir]]]]
   
        [[[weekwindvec]]]
            [[[[windvec]]]]
                plot_type = vector

        [[[weekradiation]]]
            [[[[radiation]]]]
           
    [[month_images]]
        x_label_format = %d
        bottom_label_format = %m/%d/%y %H:%M
        time_length = 2592000    # == 30 days
        aggregate_type = avg
        aggregate_interval = 10800    # == 3 hours
        show_daynight = false
       
        [[[monthbarometer]]]
            [[[[barometer]]]]
       
        [[[monthtempdew]]]
            [[[[outTemp]]]]
            [[[[dewpoint]]]]
       
        [[[monthtempchill]]]
            [[[[windchill]]]]
            [[[[heatindex]]]]
       
        [[[monthrain]]]
            yscale = None, None, 0.02
            plot_type = bar
            [[[[rain]]]]
                aggregate_type = sum
                aggregate_interval = 86400
                label = Rain (daily total)
       
        [[[monthpond]]]
            yscale = None, None, 0.5
            [[[[extraTemp1]]]]
       
        [[[monthrx]]]
            [[[[rxCheckPercent]]]]
       
        [[[monthwind]]]
            [[[[windSpeed]]]]
            [[[[windGust]]]]
                aggregate_type = max
       
        [[[monthinside]]]
            [[[[inTemp]]]]
       
        [[[monthwinddir]]]
            yscale = 0.0, 360.0, 45.0
            [[[[windDir]]]]
   
        [[[monthwindvec]]]
            [[[[windvec]]]]
                plot_type = vector

        [[[monthradiation]]]
            [[[[radiation]]]]
           
    [[year_images]]
        x_label_format = %m/%d
        bottom_label_format = %m/%d/%y
        time_length = 31536000    # == 365 days
        aggregate_type = avg
        aggregate_interval = 86400
        show_daynight = false
       
        [[[yearbarometer]]]
            [[[[barometer]]]]
       
       
        [[[yeartempdew]]]
            [[[[outTemp]]]]
            [[[[dewpoint]]]]
       
        # Daily high/lows:
        [[[yearhilow]]]
            [[[[hi]]]]
                data_type = outTemp
                aggregate_type = max
                label = High
            [[[[low]]]]
                data_type = outTemp
                aggregate_type = min
                label = Low Temperature
               
        [[[yearwind]]]
            [[[[windSpeed]]]]
            [[[[windGust]]]]
                aggregate_type = max
       
        [[[yeartempchill]]]
            [[[[windchill]]]]
            [[[[heatindex]]]]
       
        [[[yearrain]]]
            yscale = None, None, 0.02
            plot_type = bar
            [[[[rain]]]]
                aggregate_type = sum
                # aggregate_interval = 2629800 # Magic number: the length of a nominal month
                aggregate_interval = 604800    # == 1 week
                label = Rain (weekly total)
       
        [[[yearpond]]]
            yscale = None, None, 0.5
            [[[[extraTemp1]]]]
       
        [[[yearrx]]]
            [[[[rxCheckPercent]]]]
       
        [[[yearinside]]]
            [[[[inTemp]]]]
       
        [[[yearwinddir]]]
            yscale = 0.0, 360.0, 45.0
            [[[[windDir]]]]

        [[[yearwindvec]]]
            [[[[windvec]]]]
                plot_type = vector

        [[[yearradiation]]]
            [[[[radiation]]]]
           
        # A progressive vector plot of daily gust vectors overlayed
        # with the daily wind average would look something like this:
#        [[[yeargustvec]]]
#            [[[[windvec]]]]
#                plot_type = vector
#                aggregate_type = avg
#            [[[[windgustvec]]]]
#                plot_type = vector
#                aggregate_type = max


############################################################################################

[Thomas Keffer]

Elton,

My apologies for sending you off in the wrong direction. Weewx does indeed do vector averages for tags such as $week.wind.vecdir, but not for plots of wind direction. The latter are done dumbly: just a simple average over the aggregation period of the wind direction. 

You could test whether your hypothesis is correct by not doing the aggregation for, say, the week plot. The plot will be messy, but it will tell you if the averaging process is aliasing the result.

-tk

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