Sky radiation affects numerous systems but is often under estimated. This paper is especially oriented towards its determination in hot countries for cooling purposes. It includes expressions for clear weather conditions and shows some properties and the power of the flux according to the climates. Observations in Kuwait and in the Ivory Coast support the corrections established concerning hazed skies. Correction for cloudiness is treated theoretically. The limited interest of anti-convective selective coatings is finally pointed out to warn the reader against unfeasible performances.