Observations of noctilucent clouds from Lithuania

We present an analysis of systematic visual and photographic observations of noctilucent clouds seen from Lithuania in the years 1973-2009. The main trends in the noctilucent cloud occurrence frequency and the mean brightness are derived from statistical and correlation analysis. A clear signature of the solar activity cycle is imprinted on the noctilucent cloud occurrence frequency and mean brightness, both showing distinct anti-correlation with the sunspot numbers; however, no statistically significant increase of either noctilucent cloud occurrence frequency or brightness has been detected at least over past 19 yr (1991-2009). The only statistically significant positive trend is established for the numbers of very bright noctilucent cloud displays in the years 1973-2009. The most recent noctilucent cloud observations are linked to variations of local mesospheric temperatures, measured by the Aura satellite.