The Hay and Davies diffuse model divides the sky diffuse irradiance into isotropic and circumsolar components. Horizon brightening is not included. An anisotropy index, , is defined as:
The Hay and Davies model formulation for sky diffuse radiation is:
This model is implemented in the PV_LIB Toolbox as the function: pvl_haydavies1980.
-  Loutzenhiser P.G. et al. “Empirical validation of models to compute solar irradiance on inclined surfaces for building energy simulation” 2007, Solar Energy vol. 81. pp. 254-267
-  Hay, J.E., Davies, J.A., 1980. Calculations of the solar radiation incident on an inclined surface. In: Hay, J.E., Won, T.K. (Eds.), Proc. of First Canadian Solar Radiation Data Workshop, 59. Ministry of Supply and Services, Canada.
Content contributed by Sandia National Laboratories