Direct radiation: Ratio between horizontal and tilted surfaces

Abstract The ratio between direct radiation on a tilted surface and direct radiation on a horizontal surface is examined on a daily and monthly average basis. This direct ratio factor, often called R b , is shown to vary with beam intensity. The atmospheric weighting model, presented in a previous paper, is shown to accurately estimate the direct ratio factor. A comparison is made between R b calculated from hourly beam data from the Pacific Northwest, and R b predicted using the atmospheric weighting model and other direct ratio models.