The (variable) transparency of our atmosphere regulates the amount of solar energy reaching the ground. Also known as clearness index or transmissivity, it has been related linearly to relative sunshine hours ever since the seminal work of Angstr5m, whose regression model was Iater modified and generalized, to become a classic too! in the statistical assessment of the global radiation at the surface of the Earth. In this paper we submit a physico­meteorological rationale for the polynomial regression between clearness index and relative sunshine duration.