fine@psuvax.UUCP (02/23/84)
The following explanation appeared recently on the net: Clouds are white because of *diffraction*? No, they are white because they do not selectively absorb or reflect any particular color, but rather reflect all visible wavelengths with about equal efficiency. I had a radiative transfer course, offered by the meteorology department, during the fall. The professor mentioned the above as the standard explanation for clouds being white. He went on to say, however, that he thought the lack of absorption by cloud droplets was more important than the lack of selective extinction (combination of scattering and absorption). Only a small amount of radiation is absorbed, so most of the light which enters a cloud leaves it. Since white light enters a cloud, white light leaves it. Also, since photons are scattered many times before leaving a cloud, the photons leave a cloud through most (if not all) of its surfaces. Thus, clouds appear white. To support his explanation, the professor pointed out cases where individual particles produced selective extinction but the conglomerate still appeared white: Milk globules scatter blue light more than they scatter red light, but a glass of milk appears white. Colored glass ground into small bits and gathered in a pile appears white. In general, any optically thick collection of weakly absorbing particles will appear white. Other examples are salt and snow. Steve Fine