Sunshine for any point is assessed using the model representation of cloud layers to decide how much direct solar (shortwave) radiation reaches the Earth's surface. The accumulated sunshine duration at each point is calculated from the length of time for which the direct solar (shortwave) radiation at the Earth's surface, falling on a plane perpendicular to the direction of the Sun, is greater than or equal to 120 W m⁻². The minimum solar intensity level of 120 W m⁻² is defined by the World Meteorological Organisation and is consistent with observed values of sunshine duration from a Campbell-Stokes recorder (sometimes called a Stokes sphere) which can only measure moderately intense sunlight and brighter.
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). But, it is suspected that currently the amount of sunshine is over-estimated because the amount of cloud cover is only an average over the whole grid box and cloud is not necessarily uniformly distributed in time or coverage.
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