Abstract
Evidence is given for a secular change now taking place in the frequency of occurrence of noctilucent clouds. Separate lines of argument lead to the strong supposition that this change occurs as the result of a small, systematic cooling of the upper mésosphère in summertime. The change is likely to have amounted to 7 K over the last 20–30 years. While changes in water vapour concentration will affect the frequency of occurrence, it is just as likely that the changes may be taking place in the mean mesopause temperature. These changes in mean temperature increase the probability of occurrence of a low (threshold) temperature which allows cloud formation.
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