Direct solar radiation on the ground

The spectral transmissivity rA from (5.110) also gives the ratio of the spectral irradiance EXn of an area located on the ground and oriented perpendicular to the direction of radiation, to the spectral irradiance Esx by extraterrestrial solar radiation. It therefore holds that 

In most practical applications, it is normally sufficient to know just the irradiance E at an area on the ground. The most exact method for obtaining E is by integrating the spectral irradiance Ex over all wavelengths, practically from A = 0.3 /im to A = 4.0 /im. In order to avoid this somewhat difficult numerical 

A rather accurate calculation of E is permitted by the relationships given by R.E. Bird and R.L. Hulstrom [5.42] for the irradiance of direct (directional) solar radiation. The irradiance of a surface, whose normal forms an angle /3s with the sun s rays, is 

The utilisation of replaces the estimation of the parameters a and /3. Absorption by Ozone12  

Example 5.7 Determine the irradiance of direct solar radiation on a horizontal area in Berlin on 1st September, 12.00 central European summertime, cf. Example 5.6. The sky is cloudless, ho3 = 0.30cm, w = 2.6cm and a horizontal visibility of h = 40km may be assumed. 

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