noctilucent clouds (NLC)

A high-atmosphere phenomenon appearing as luminous bluish clouds in the summer twilight sky. They occur at heights of about 80 kilometres (50 miles) and show a range of structures. Noctilucent clouds are very thin and scatter only a very small proportion of the sunlight incident on them, so they cannot be seen from the ground during daytime or in bright twilight. Since they are a summer phenomenon, they cannot be observed from the highest latitudes, where the sky never gets dark enough. However, they are also a high-latitude phenomenon, so the range of latitudes from which they can in practice be seen, between 50° and 65°, is quite narrow. The clouds form in the presence of nuclei on to which water can condense and freeze into ice. It is not known just what these nuclei are, but they may be ions created by solar ultraviolet radiation or micrometeoritic particles. The main prerequisite is a low enough temperature, which is calculated to be 120 K (-150°C) at a height of 80-90 kilometres (50-56 miles). This is created by the pole-to-pole air flow at these heights and is not affected by the level of solar radiation. There is observational evidence to suggest an overall increase in the occurrence of NLCs over recent decades, and this has been linked to an increase in water vapour in the upper atmosphere as a consequence of increasing amounts of methane. The number of sightings also varies inversely with the solar activity cycle.