Three dimensional radiation effects
Incident solar radiation includes direct beam, scattered diffuse and by the surroundings reflected radiation. The largest challenge for the determination of solar radiation poses cloudiness, especially scattered cloudiness. Clouds lead to scattering and reflection of the incident shortwave radiation. Under some conditions, when the sun is not obstructed by clouds, global radiation may be even higher that under cloudless conditions. This "radiation enhancement" by clouds is due to reflection by the surrounding clouds towards the observer. Topography is the second factor which leads to a complex three dimensional radiation field. Surrounding mountains may lead to shading, and multiple reflections of the incident solar radiation. The surrounding mountains also emit longwave radiation which leads to an increase in radiation balance. Threedimensional radiation models help to better understand these phenomenas.