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 decades the interest they have been given has markedly increased. The clouds, usually pale blue in colour, may be seen on a clear night at high latitudes. Typical examples are illustrated in the book....