Solar Radiation Estimation From Cloudiness Data. Satellite Vs. Ground-Based Observations

Solar radiation is estimated from ground-based observations of point cloudiness C and hourly cloud fraction n data provided by Meteosat satellites. Two types of clear sky/overcast sky are defined, associated to C = 0/1 and n = 0/1, respectively. The number of hours on n-clear sky is three times larger than on C-clear sky. On C-overcast skies, the regression relationships for solar global irradiance are slightly more accurate than those for clearness index. The average irradiance on C-overcast sky is about a quarter of that on C-clear sky. The average irradiance on n-overcast sky is about one-third of that on n-clear sky.