Soiling Events and Anomalies
Close inspection of the sample sites in the preliminary study showed that about half exhibited a “grace period” after the last spring rain where soiling was negligible, or a period during which soiling rates appeared slower than the later part of the dry season. Figure 1 illustrates this effect. The last significant rainfall of the spring occurred in late April, and the rate of performance decline did not start to increase appreciably until the middle of June. Observed grace periods for those sites that exhibited this behavior was 20~50days. However, just as many sites did not exhibit grace periods.