How to interpret cloudbase map when data appears to be blocky or incomplete.

  • How should I interpret the cloudbase map when the data appears to blocky, patchy and/or incomplete as shown in the example below using the ECMWF model.
    This image is for central east coust of New Zealand's North Island. It is for approximately 6 days in the future. I am unsure how to interpret the very straight, defined edges. Particularly where the cloud base seemingly goes from 0m to unlimited almost instantanoeusly.
    In comparison, about 10 hours later the model looks like this;
    The regions look much more organic with only a few patches that appear to be missing.

    Am I correct to interprete these square edges as missing/incomplete data?

    If this is true, how can I determine whether an uncoloured region in any particular instance on the map is because there is no cloud base or simply that there is no data available? This would seem to make interpreting the cloudbase very unreliable.

    Would it be possible to indicate areas of no-data with another shading to differentiate between this and where there is truely no cloud forecast.

  • Moderator

    @NevP This is a limitation of the weather model and a slight error with interpolation. The colours should be blended together a little smoother, sometimes it bugs out like this. Missing boxes in large areas of colour should be interpreted similarly to surrounding areas of colour.

    Hope this helps.

  • Sailor Moderator

    If you look at the colour scale of cloud base in Settings, you see that grey corresponds to cloud base higher than 8000m OR that there is no cloud. To check that, just compare the Cloud base layer and the Cloud layer. Where is no cloud, there is no cloud base.


    Then, why does this layer look patchy with geometrical shapes? This is because there is no interpolation between each grid point of the model. It gives the same type of image as the Precipitation type layer. The image is not smoothed to avoid mixing of the different levels .
    Some additional information:

  • @NevP

    I created a vastly better, more detailed and far easier (and more accurate) to interpret cloudbase overlay setting, which you can download from here (have some patience as it will take a few seconds for this page to load due to the amount of text settings and graphics within it:

    The specific directions to install the custom overlays is found within the first post in this custom graphics overlays thread, here:

    Examples and a legend to interpret it:

    Cloud Base Legend.png

    This is the custom cloudbase overlay:

    This is the standard Windy overlay:

    Current time forecast for detail and info comparison:

    My cloud tops overlay also uses these same color shades and altitude divisions so that a pilot can easily interpret and assimilate both for any required altitude range.

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