Recent studies of noctilucent clouds indicate a secular increase in their frequency of occurrence which may be associated with changes either in the water vapour concentration or in the mean temperature of the mesopause. Noctilucent clouds are expected to be seen very close to the level where the relative humidity is 100% and changes in temperature of the mesosphere or in its water vapour content might be expected to alter the height at which the clouds occur. Comparison of our height measurements with those obtained over the past 100 yr shows that the suggested changes since 1885 in temperature (a decrease of approximately 7°C) and water vapour mixing ratio (an increase of approximately 1.7 ppmv) have not been sufficient to affect the height of the clouds by an observable amount. Future height measurements of noctilucent clouds may provide a practical indicator of changes taking place in the mesosphere which could be associated with global-scale alterations of the lower atmosphere.