We present the results of experiments which demonstrate that a sinusoidal variation in the long-term, STC-corrected, outdoor performance of PV modules is caused by seasonal spectral changes in the received sunlight. This variation may be factored out to increase the precision of outdoor studies. We use this result (a) to quantify the rate of EVA degradation observed in the Negev desert under natural 1-sun conditions and (b) to identify a principal source of the "summer recovery" observed in modules of amorphous silicon cells.