Sky clearness index was used to classify the type of sky into
clear, partly overcast and overcast conditions. Heavy overcast sky
corresponds to a sky clearness index equal to or less than 0.2.
Sky clearness index is between 0.2 and 0.6 when the sky is partly
overcast, and the value is over 0.6 under clear sky. Data obtained
when solar altitudes were less than 4◦ were not analysed in this
study. Sixty seven percent of the monitored period was classified
into heavy overcast sky and 31% of the period was partly
overcast sky. Clear overcast sky accounted for only 2% of the
period.
Horizontal illuminance ranged from 5317 lx at 6 a.m. to 53,904 lx
at 12 p.m. Illuminance on an east-facing plane (azimuth angle of
94◦) reached a peak of 25,648 lx at 9 a.m., then gradually reduced.
The highest illuminance on a west-facing plane was 38,429 lx at
3 p.m., while the illuminance on a north-facing plane was 14,345 lx
at 12 p.m.