At latitudes above 50°, the summer nights are quite light; at the solstice civil twilight lasts all night for latitudes between 60.6 and 66.0°. The conditions are not good for astronomical observations but can be excellent for twilight exploration of the upper atmosphere.
At high latitudes and heights around 80–90km, the atmosphere is very cold during the summer; paradoxically, this part of the upper atmosphere is coldest in summer and warmest in winter. Although the upper atmosphere is known to be dry, with a water vapour pressure amounting to just a few millionths of the local air pressure, clouds are seen there. These clouds shine in the summer twilight sky with a pale blue colour and their occurrence, shapes, and position in the sky have been observed and noted for over one hundred years (the first published note was by Leslie1 in 1884).
Paton,2 then head of the BAA’s Aurora and Zodiacal Light Section, began annual reports in 1964. The Associaion now possesses an unbroken series of observations which have been listed in some detail in the Meteorological Magazine3 until 1985. Shorter annual reports are now being published; at first they continued to appear in the Meteorological Magazine but now appear here in the Journal. Gavine’s most recent summary of noctilucent cloud observations4 includes a pair of very beautiful colour pictures taken by Holger Andersen in Denmark. Original reports and photographs from recent years are stored in the Archives and Special Collections of Aberdeen University (MS3152/24/) where they are freely available for examination.
Because of the extremely low water vapour pressure in the 80–90km region of the atmosphere, it is clear that nucleation of clouds cannot occur there unless the air temperature is very low. Extremely low temperatures (sometimes below 110K, -163°C) have been measured in rocket soundings over northern Norway during summer months.
The ice crystals in the clouds begin to grow on nuclei, which may be ions or meteoric smoke particles, only at the very lowest temperatures which are encountered at around 90km height. During the summer, the winds at these heights flow out from the pole and the geostrophic wind direction is, therefore, generally towards the southwest. The ice crystals take many hours to develop into an observable cloud and during this time the clouds are swept many hundreds of kilometres out from and around the pole.
Observations from satellites disclose that there is a semipermanent cloud layer surrounding the summertime pole and noctilucent clouds form the outer parts of this cloud. As the cloud particles grow in size, they fall at an ever-increasing speed until eventually they drop into a region of the atmosphere where the air temperature is high enough for the air to be unsaturated. The particles then begin to evaporate and lose mass; their fall speed decreases, resulting in an abrupt disappearance of the cloud. NLC appear in a horizontal layer at an air temperature which seems to be always close to 150K (-123°C), according to the rocket soundings of Lübken, Fricke and Langer.5 The visual observations suggest the final stage in the life of a noctilucent cloud is reached at latitudes around 55–65°. It would be interesting to know if the latitude of this edge has decreased in the past thirty years and if it is decreasing at the present time.
The 32 years of data show that the clouds occur principally within a month of the solstice. They are most likely to be seen on the night of July 3/4; the earlier decile (90% of the sightings occur after this date) is June 5/6 and the later decile (90% of sightings earlier) is July 31/August 1. Spaceborne photometers have shown that the seasonal variation is a frue surmner maximum, for in the southern hemisphere the clouds are most frequently seen in early January. The data have also shown that the frequency of occurrence of the clouds, that is to say the total number of nights on which the clouds are seen in any one year, has been rising over the last three decades and they are now twice as likely to be seen as they were in the 1960s. The reason for this change has been discussed in a number of papers.6, 7 The year-to-year numbers also show an obvious 10.5 year variation8 which could be the effect of solar activity on the temperature of the upper atmosphere. Also, the numbers of observations in 1992 and 1993 were obviously low and this is probably the effect on the upper atmosphere of the volcanic eruption of Mount Pinatubo in July 1991. Satellite observations9