A model of low-energy scattering of protons and neutrons that combines the shell model and the compound nucleus model of nuclei. In the cloudy crystal ball model, the incoming nucleon is taken to interact with the nucleus via a complex potential. By analogy to optics, the real part of the potential is associated with the propagation of the nucleon through the nucleus, while the imaginary part is associated with absorption of the nucleon. The real part is an average field that is similar to the potential of the shell model. The limit of strong absorption corresponds to the compound nucleus model. The cloudy crystal ball model was proposed by Herman Feshbach, Charles Porter, and Victor Weisskopf in 1954 and has been used with considerable success ever since. It explains how a nucleon inside a nucleus can have a long mean free path, thereby enabling the shell model to be viable.