The secular changes in noctilucent cloud occurrence: Study of a 31-year sequence to clarify the causes

The frequency of occurrence of noctilucent clouds observed from northwest Europe varies markedly and systematically from year to year. Ground-level data, acquired by a dedicated network of observers, are available in much detail for each year from 1964 to 1994. There has been a noteworthy increase in the frequency of noctilucent clouds over these thirty one years and there is a superposed modulation of 10.5 year period. The data of earliest appearance at dusk and latest disappearance in dawn suggest that these changes do not result from changes in typical brightness of noctilucent clouds. (C) 1997 COSPAR. Published by Elsevier Science Ltd.