Noctilucent Clouds

An atmospheric phenomenon is considered as ''explained'' when we have succeeded in deducing it on the basis of accepted principles of physics. (H. Ertel, Methods and problems of dynamical meteorolo­ gy,p. l) Until recently, noctilucent clouds were regarded merely as a curious atmos­ pheric phenomenon, the occurrence of which aroused only limited scientific attention. However, in the last two decad...