Estimating daily global radiation from air temperature and rainfall measurements

The model first assesses days on which the probabilities of clear and overcast skies are maximum and predicts, for these days, the mean sky transmittance. The mean sky transmittance can then be interpolated for any other day. In the model, station parameters are directly derived from latitude and ancillary information, so neither historical climatic records not calibration parameters are required for the calculations. This simplifies to a large extent the estimation procedure. The model was tested for 22 weather stations in 11 countries. Measured and estimated records were also separately used as input of a wheat growth simulation model (NWHEAT). When measured irradiance is substituted by estimates there is a very small deviance in the grain yield as calculated by the model.