A Method of Calculating the Direct Shortwave Radiation Income of Slopes

Abstract

A formula has been developed whereby variations in the shortwave radiation income on different slopes in any latitude can be easily determined from a knowledge of surface geometry and the sun's declination. This is achieved by expressing slope characteristics and the sun's position as unit vectors in the coordinates of a common system, and multiplying the cosine of the angle between the two vectors by a factor involving the solar constant, atmospheric transmissivity and the optical air mass. Integration of the formula permits the topographic variations of direct shortwave radiation income over specified areas to be calculated for daily or longer periods from radiation observations at a single site. Thus, a device has been developed which has applications in many aspects of pure and applied microclimatology, and which differs from other similar formulae in that it is at the same time both integrable and applicable under different atmospheric transmissivities.