There were some spectacular evening twilights this week. Long after the sun had sunk below the horizon, the sky was bathed with a phenomenal orange-yellow glow, far brighter and longer-lasting than the usual twilight.

Such amazing evening light can sometimes come from volcanic eruptions, when dust high in the atmosphere acts like a cinema screen, bouncing back light from the Sun below the horizon. But there were no recent eruptions that could explain this week’s twilights.

The mystery may have been solved by the physicist Les Cowley, an expert in atmospheric optical phenomena. He believes that the Sun’s rays were being reflected by extremely high clouds in the stratosphere, about 20km (12 miles) high, far above the height of normal clouds. Usually the stratosphere is far too dry for clouds to form, but when it is intensely cold, below minus 70C (-94F), the extremely low temperatures can produce clouds of acid. Such clouds usually form only in the intense cold of winter in the Arctic or Antarctic, but this week there was exceptionally cold air in the stratosphere over Europe.

Despite the glorious evening light shows, these unusual clouds help chemical reactions in the stratosphere that eat into the Earth’s ozone layer, producing ozone holes that allow ultraviolet light to reach the Earth’s surface.