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IFS sunshine values are derived using the mean of the direct solar radiation reaching the surface over the whole grid box.  Thus sunshine values are reasonable If there is continuous full cover of cloud (zero sunshine) or cloud-free skies (sunshine totals approaching the maximum for the latitude and time of year).  Currently, partial cloud cover over the grid box reduces direct sunshine accordingly (e.g. 50% cloud cover implies only half of the day's available sunshine will be predicted).   Currently the amount of sunshine is suspected to be an over-estimate.  This is because cloud is not necessarily uniformly distributed in time or coverage over the whole grid box.

Current meteograms include total cloud cover.  However, sometimes there may be complete cover of high cloud cover but still the sun is shining and the perception by users can be very different from overcast conditions.  Consideration should be given to the density or thickness of high cloud in assessing the whether the forecast should be for bright or sunny conditions rather than for gloomy and overcast conditions.