Three alternatives to the existing ASHRAE clear sky irradiation model are offered to improve the accuracy of the predictions and the range of application. In rising order of complexity, these alternatives are:

• Revised constants A, B, and C, which take into account the advancement in solar radiation research over the past two decades.
• An easy algorithm that uses the horizontal visibility at ground level as a parameter of tur­bidity instead of monthly constants in conjunction with clearness numbers.
• Same as the former with an additional humidity term to account for variable water vapor absorption.

The proposed changes will improve the accuracy of the model; the latter two will also transform the ASHRAE model into a truly universal model, not restricted by regional peculiarities.