In the present study a mathematical model is proposed for estimating daily clear sky solar radiation in Thessaloniki. The relation I = A*sin (h) + B was used for calculating the intensity I of solar radiation. The coefficientsA andB depend on the quantities of water vapour and aerosols in the atmosphere and the sunheight (h).A andB have been estimated separately for each month. The application of the proposed model is only possible when the values ofA andB and two other astronomical parameters (sun's declinationδ and hour angleH) are known. A good agreement has been found between observed and computed values, a fact significantly reinforcing the accuracy of the proposed model.