Peter Dalin (IRF)Date:
AniaraThe mystery of noctilucent clouds
Swedish Institute of Space Physics
Noctilucent Clouds (NLC), or "night-shining" clouds, are the highest clouds in the Earth's atmosphere observing around the summer mesopause at 80-90 km altitudes, the coldest place in the atmosphere. These night clouds are comprised of small water-ice particles because of the extremely low temperatures in the summer mesopause. These particles scatter sunlight and thus NLC are readily seen against the dark twilight sky. The visible structure of NLC is similar to cirrus clouds but the former show more intensive and complex structures with signatures of waves of different scales. The basic physics of NLC formation is rather well understood at present time. However questions concerning secular trends in NLC characteristics (or global change effects), the relationship between NLC and solar activity, differences in the NLC statistical behavior between different observational sites and many others are still unanswered. The main problem of current investigations in the field of NLC research is a high degree of the NLC variability. It concerns both long-term (year-to-year) periodicities and short-term (during a season) variations.
In this study, we conduct a statistical analysis of the relationship between more than 40 years of variations in noctilucent clouds and lunar orbital motion. Monthly (29.53 days) and semi-monthly (14.77 days) components related to changes in the lunar age generate modulation in the probability of noctilucent cloud occurrence of about 20% and 10%, respectively, with a statistical significance of 99%. Changes in the distance between the Moon and Earth influence the frequency of the occurrence of noctilucent clouds: the probability of
observing these clouds is greater when the Moon is close to perigee and apogee. This provides one more mystery in studies of noctilucent clouds. Created 2006-05-23 08:53:00 by Uwe RaffalskiLast changed 2006-06-07 15:26:34 by Mats Holmström