As summer arrives in the northern hemisphere, so do the noctilucent clouds – hopefully. These high-altitude formations are as enigmatic as they are beautiful. Their name derives from Latin, meaning “night shining”.

They appear during the summer months and glow with an electric-blue intensity against the darkening western sky. Look for them about half an hour after sunset.

Existing at an altitude of about 80km, virtually the edge of space, they are the highest known clouds in Earth’s atmosphere.

Some believe the key to understanding the clouds is that there appear to be no recorded sightings of them until 1885. Considering how obvious they are, some researchers have suggested that they may be the result of industrial pollutants that were beginning to really accumulate in the atmosphere by the late 19th century. These aerosols then provided sites around which the water ice could form.

Others point out that the timing of their appearance roughly coincided with the 1885 eruption of Krakatoa. Still others think it is the way that climate change drives water vapour into the upper atmosphere that makes the clouds possible.

Whatever their origin, they are beautiful formations to watch out for. In the southern hemisphere, the noctilucent cloud season begins around October.