Late spring, early summer marks the beginning of noctilucent cloud season in the northern hemisphere. The name derives from Latin, where noctilucent means “night shine”. These beautiful cloud formations can often be seen during the summer months shining with an electric blue colour against the darkening western sky about 30 minutes after the sun sets.
The origin of the noctilucent clouds remains mysterious. They are the highest known clouds in Earth’s atmosphere, existing at an altitude of about 80km (50 miles), which is virtually the edge of space. They are regarded as being too high and too tenuous to have any effect on the weather at ground level.
A puzzling aspect is that there appears to be no recorded sightings of the noctilucent clouds before 1885. This seems strange considering how obvious they are. Some have suggested that perhaps they form when water freezes around industrial pollutants, which were first released into the atmosphere during the Industrial Revolution. Others suspect it is because the atmosphere now contains more of the greenhouse gas methane, which promotes the production of water vapour in the upper atmosphere.
In the southern hemisphere, the noctilucent cloud season begins around October.