|Noctilucent clouds (NLC) is a speciality at latitudes 55-65N (and south, too). On northern hemisphere phenomenon is visible occasionally in May and more frequently in June. However, the real activity is observed in July and in early August at my latitude of 60N. During those months our atmosphere on northern hemisphere experiences the temperature minimum 130 K, which is low enough water vapor to condense into clouds at the extreme height of 82-84 kms (and at the extremely low pressure).
Below is short statistics of my NLC-observations. Columns are: number of sightings each year, the earlist sighting, sightings sorted by month, the latest sighting and relevant extra info if any. In general, it is not possible to state anything about the frequency of the NLCs each year based on the given data below. Summer is the time to make travelling and so there are gaps in observing. Moreover, weather is a major issue in observing. For a more comprehensive listing see www.ursa.fi where you can find all Finnish NLC-observations.
|Weather & Misc. phenomena root
|Stars & planets & Night sky root