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This paper reports the application of a non-linear relationship between the incoming shortwave solar radiation and bright sunshine duration. The newly suggested equation is a modified form of the existing Angstrom equation. Measurements of solar radiation and sunshine radiation from 1997 to 2006 at 21 meteorological stations were used to calibrate and validate the suggested equation. The model parameters required to specify the nature of the relationship between solar radiation and sunshine duration were determined by automatically minimizing the difference between the modelled and measured solar radiation. At the 21 meteorological stations, the absolute error (AE) is in the range of −0.126-0.158 MJm −2 day −1 for the original Angstrom equation, while it is in the range of −0.089-0.154 MJm −2 day −1 for the modified equation. The root mean square error (RMSE) is also improved by 7-8% for the modified method. The results show that the newly suggested equation generally provides better performance than the existing Angstrom equation.