Comparison of Cloud Amounts Retrieved with Three Automatic Methods and Visual Observations

Four methods have been used for the estimation of total cloud amount and low clouds: visual observations, Long method applied on pyranometer measurements, Automatic Partial Cloud Amount Detection Algorithm (APCADA) pyrgeometers ceilometer measurements base height. Records from meteorological observers indicate that clear days (0–1 octa) represent most frequent clouds. In contrast, is more aleatory. Results obtained show maximum frequency in extreme values. The APCADA also indicates predominance cloudless skies. shows a completely skies, but cloudy (8 octas) second case. report overcast skies than observer. agree with or differ ±1 octa 60–76% cases 56–63% amount. general, agrees observer automatic underestimated values possibly due to difficulty monitoring high