Types of Iridescent Clouds

THE remarkable clouds at high altitude which have been the subject of two interesting articles by Prof. Carl Størmer 1 and Prof. S. Chapman 2 in NATURE are referred to as iridescent or mother-of-pearl clouds. In order to prevent misunderstanding, I should like to point out that the high clouds in question should not be confused with the much more common ‘iridescent clouds’ which are described in most textbooks of meteorology. 3 The latter are cirro-stratus or cirro-cumulus clouds and are certainly well within the troposphere, at a height of approximately 10,000 metres, while the clouds described by Størmer are two or three times as high and are well within the stratosphere.