Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces

A detailed comparison was made between five simple broadband models for clear sky global insolation. Compared models were those of Atwater and Ball, Davies and Hay, Watt, Hoyt, and Lacis and Hansen. A sixth simple model, called the Bird model, has been formulated by using parts of these five models and by comparison with the results from three rigorous radiative transfer codes. All of the simplemodels provide results that agree within <10% with the three rigorous codes when the sun is in the zenith position. The Bird and Hoyt models agree within 3% with each other and with the results of the rigorous codes. However, the Bird model is easier to implement and has broader application than the Hoyt model.