Late spring/early summer in the northern hemisphere is a good time to look for noctilucent clouds. Roughly translated from Latin, noctilucent means “night shine”, which is a good description of these beautiful cloud formations. They can be seen glowing an electric blue colour against the darkening western sky about 30 minutes after the sun sets.
Noctilucent clouds are the highest known clouds in Earth’s atmosphere, existing at an altitude of about 80km, which is virtually the edge of space. The Kármán line, which marks the boundary between the atmosphere and outer space, is defined as being 100km in altitude.
It is not clear how noctilucent clouds form. As with all clouds, water vapour must condense around a “nucleus”, usually a dust grain. The problem is understanding how such material gets that high in the tenuous upper levels of the atmosphere. It has been suggested that the disintegration of meteors produces the dust.
Another possibility is that human-made pollutants reaching the upper atmosphere are acting as nuclei, enhancing the effect over recent centuries. If industrial pollution is to blame, this may explain why there appears to have been no reported sightings of noctilucent clouds before 1885.
