Noctilucent or 'night shining' clouds are a summer, polar phenomena, most commonly observed at latitudes between 55 and 65 degrees. The clouds can only be seen when the sun is shining on them (at about 83 km altitude) and not on the lower atmosphere (when the sun is between 6 and 16 degrees below the horizon). They form in the polar mesopause, which is the coldest region of the Earth's atmosphere, reaching temperatures as low as −140°C.
Noctilucent clouds were first reported in 1885 when they were independently observed in Germany and Russia. This was two years after the volcanic explosion of Krakatoa in the Straits of Java.
The initial observation of noctilucent clouds was thought to be related to an increase in the number of observers of the twilight skies attracted by the spectacular displays resulting from the globally distributed volcanic debris of Krakatoa. Alternatively, water vapour injected into the upper atmosphere by the volcano ultimately reached the cold, dry upper mesophere.
Subsequent observations have proved that noctilucent clouds are not solely related to volcanic activity, and their volcanic association is now scientifically contentious. It has been alternatively claimed that the appearance of noctilucent clouds is the earliest evidence of anthropogenic climate change. Noctilucent cloud observations from north-west Europe over the last 30 years show an increasing trend in the number of nights on which the clouds are observed each summer season, superimposed on a decadal variability that appears to be solar-cycle related.
Competing anthropogenic explanations for this increasing occurrence of noctilucent clouds focus on either excessive greenhouse cooling of the middle atmosphere or increased water vapour linked to increased methane release associated principally with intensive farming activities.
The first image in this page's gallery was taken from Davis in Feb 1998. The noctilucent cloud is wavy and blueish-white. The picture was taken when the sun was approximately 10 (degrees) below the horizon, but still shining on the noctilucent cloud.
The clouds have been observed thousands of times in the northern hemisphere, but less than 100 observations have been reported from the southern hemisphere. It has not been resolved if this is due to inter-hemispheric differences (temperature and/or water vapour) in the atmosphere at these altitudes, or the lack of observers and poorer observing conditions in southern latitudes. This is a subject of Australian Antarctic Division study.