Investigation of the characteristics of the reflectivity of clouds

This paper investigates some of the reflectivity characteristics that cloud s (when modelled as solid bodies) must exhibit to be compatible with observ ations that the reflecting surface of a cloud (i) appears almost equally br ight across its face, (ii) is brightest when the cloud is opposite to the S un but decreases in brightness as the cloud moves to other positions and (i ii) increases in brightness with increasing optical thickness of the cloud in the observer's line of sight. These observations, respectively, are show n to imply that the peak value of the bidirectional total reflectivity from a cloud surface (i) increases in inverse proportion to the cosine of the a ngle between the Sun and the normal to the cloud surface, as the incident a ngle increases, (ii) appears to be directed back in the direction of the in cident radiation, and (iii) increases as optical thickness of the cloud in the observer's line of sight increases. The results could have application in many fields (e.g. modelling diffuse radiance distributions for cloudy sk ies).