THE IMPORTANCE OF THE SAMPLING FREQUENCY IN DETERMINING SHORT-TIME-AVERAGED IRRADIANCE AND ILLUMINANCE FOR RAPIDLY CHANGING CLOUD COVER

The sampling interval is an important parameter which must be chosen c arefully, if measurements of the direct, global, and diffuse irradianc e or illuminance are carried out to determine their averages over a gi ven period. Using measurements from a day with rapidly moving clouds, we investigated the influence of the sampling interval on the uncertai nty of the calculated 15-min averages. We conclude, for this averaging period, that the sampling interval should not exceed 60 s and 10 s fo r measurement of the diffuse and global components respectively, to re duce the influence of the sampling interval below 2%. For the direct c omponent, even a 5 s sampling interval is too long to reach this influ ence level for days with extremely quickly changing insolation conditi ons.