A magical sight appeared just before dawn in Northern Ireland on May 5. A spooky, electric-blue glow lit up the northern sky; it was seen independently by two astronomers searching for meteors.

This fantastic spectacle was an unusually early case of noctilucent, or night-shining, clouds. These are the highest clouds in the atmosphere, floating around 80km (50 miles) high in the mesosphere, on the edge of space. Being so high, they are usually masked from view, so the best times to see them are in late twilight, after sunset or before dawn, when they often appear as shimmering, silvery blue ribbons. And usually the best months to see them are in June and July. “In my long years of observing noctilucent clouds, this is the earliest I have ever seen them,” reported John McConnell at Maghaberry, Co Antrim.

Noctilucent clouds are unlike normal clouds. In the mesosphere temperatures drop to minus 130C (-200F) with 200,000 times less atmosphere than the air we breathe. When the clouds were first sighted they were thought to come from volcanic ash, following the eruption of Krakatoa in 1883. But they have carried on appearing ever since, and in recent times have grown more intense and widespread.