Night owls out and about during the long twilight hours of midsummer might have noticed something strange recently.

Out over the northern horizon, strange glowing clouds shimmer when all else is dark. They are noctilucent, or 'night shining' clouds, floating serenely some 50 miles (82km) above the earth's surface.

Anglesey scientist John Rowlands, who took the photo above, explains: "Noctilucent clouds are fantastically beautiful and may be telling us something important about our atmosphere.

"They had never been seen before the mid-19th century, and so it is believed that these eerie clouds are forming due to methane making it into the upper atmosphere - an area way above normal weather clouds, where it should in fact be bone-dry.

"We believe methane migrates to the mesosphere and breaks down into water under the action of sunlight. The methane could well be coming from tundra that is beginning to melt in the subarctic regions of the world."

Anglesey is one of the few places in the UK where this phenomenon can be seen clearly, because light pollution is still fairly low compared to other parts of the country.

This image shows the view on 16 June 2009, when the clouds were exceptionally bright and stretched from horizon to horizon in huge billowing waves and whispy lines.

"Have a look out for them from about 11pm onwards over the next few weeks" says John. "You'll never see anything quite like noctilucent clouds."