On the relation between global insolation on horizontal and tilted surfaces

Abstract

Global insolation has been measured at many sites on a horizontal surface, but is needed on a tilted surface. A study has been made of patterns of global insolation for 12 months at Fort Hood, Texas, where measurements are made on both horizontal and tilted surfaces. The results indicate that, during clear weather, use of a geometrical formula for converting horizontal to tilted insolation for direct rays result in values 3.3% high. For the twelve month period March, 1976–February, 1977, including all weather, but only hours where the insolation on the tilted surface is greater than 22 Langleys/hr, the geometrical formula is 4.6% high. It is concluded that the geometrical formula can be used with small corrections.