It’s time to watch for the beautiful cloud formations known as noctilucent clouds. Latin for “night shining”, noctilucent clouds are seasonal and become visible in the northern hemisphere from late spring/early summer.
Although unpredictable, when they do appear they will be seen in the western sky about half an hour after the sun sets. As the sky darkens into night, they will shine with an extraordinary electric blue colour.
Part of the appeal of these rare clouds is that their origins are still mysterious. They are the highest known clouds in Earth’s atmosphere, condensing at an altitude of about 80km (50 miles). This is virtually the edge of space. Their extreme altitude is what allows them to catch the last rays of the setting sun, even when Earth’s surface beneath them is already in darkness.
Perhaps the most puzzling aspect of the clouds is that there are no recorded sightings of them before 1885. Some suggest the clouds are therefore caused by industrial pollutants to which water vapour freezes, or by the rise of the greenhouse gas methane in the atmosphere, which promotes water vapour production in the upper atmosphere.
In the southern hemisphere, the noctilucent cloud season begins around October.
