Abstract
Noctilucent clouds are tenuous ice clouds that form in the upper mesosphere over arctic and subarctic regions in the summer. Observations of the clouds made during 1988 and 1989 by the North American surveillance network NLC CAN AM are presented. Observing took place from 15 May to 15 August, except for arctic sites, which observed from 1 April to 30 April and 15 August to 15 September due to the Midnight Sun. The peak incidence of noctilucent clouds occurred in July during both years. Both positive and negative sightings, that latter determined by at least two readings under favourable conditions, are plotted on date-latitude graphs. An apparent northern migration of noctilucent clouds (NCL) with latitude as the season progresses was detected. Although an attempt was made to look for an apparent longitudinal drift, none was found. A detailed comparison of how NLC have behaved and changed in the past quarter century is examined using the NCL CAN AM data in conjunction with the 1964-65 data presented by Fogle (1966).Key words: noctilucent clouds, northem migration, longitudinal drift, comparison
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