Sunshine duration is the length of time that the ground surface is irradiated by direct solar radiation (i.e.,
sunlight reaching the earth's surface directly from the sun). In 2003, WMO defined sunshine duration as
the period during which direct solar irradiance exceeds a threshold value of 120 watts per square meter
(W/m2
). This value is equivalent to the level of solar irradiance shortly after sunrise or shortly before
sunset in cloud-free conditions. It was determined by comparing the sunshine duration recorded using a
Campbell-Stokes sunshine recorder with the actual direct solar irradiance.