The North-West Europe data on noctilucent clouds: a survey

There are published data from a set of observers who have been, and are, observing under an almost-unchanged protocol without interruption from 1964. These data have already been used to demonstrate that there has been a secular increase in frequency of occurrence of noctilucent clouds. There is a 10.4-year modulation of this increase which lags on the solar activity cycle. It could be that this modulation is related to a periodic variation in the height of the 30 mb pressure level, as described by Labitzke and van Loon. There is no indication that the general increase over the years arises from a change in brightness of noctilucent clouds. The seasonal variation shows no change over the last three decades; there has been no change in the length of the season concomitant with either the secular change or with the 10.4-year periodic change seen superposed on the secular change. There are no preferred days for noctilucent clouds appearance, e.g., there is no connection with meteor showers. Noctilucent clouds are seen more frequently after midnight, There is no long term change in the latitude of the southern edge to be seep in the data; there is a tendency for noctilucent clouds to be slightly further North (by 0.9 degrees) in the last third of the season. During the night, the noctilucent clouds appear to drift northwards by about 3 degrees before midnight and return southwards by the same amount after midnight. Their azimuth of symmetry in the twilight sky is 351 degrees i.e., the southern edge of noctilucent clouds appears to be aligned 9 degrees from due East-West. (C) 1998 Elsevier Science Ltd. All rights reserved.