Noctilucent Clouds

Knowledge of noctilucent clouds is reviewed, and fresh observations made in Sweden are described. The conditions under which the clouds may be seen are calculated and compared with observations; it appears that the clouds are formed only in the summer in high latitudes. The sizes and concentrations of the cloud particles are inferred, and the probsem of their composition and origin is discussed. The clouds are not likely to be formed by condensation of water vapour, but are more probably dust clouds derived from great eruptions, very large meteorites, and from interplanetary space. Evidence of dust in the stratosphere is assembled, and the processes by which it appears in visible clouds are discussed.