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Noctilucent Clouds imaged by Eero Karvinen (more images) over Finland July, 2006. ©Eero Karvinen, shown with permission.

Shining eerily blue-white long after sunset, their corrugations knots and streaks sliding and shifting by the minute, noctilucent clouds are Earth’s highest. 80-85km (50-53mile) high in the mesosphere the pressure is only 1/500,000 of that at Earth’s surface. Minute ice crystals form the clouds when the temperature falls below -123C.

The low temperatures and formation conditions are, paradoxically, reached during summer and the time to see them in the Northern Hemisphere is from mid-May to August.

High latitudes are ideal but they can be seen widely over Europe, UK, Austria, Italy and southern Germany. In the US, Utah and Colorado have had them.

They will soon be visible -- Choose a dark, moonless night and scan the northern horizon. Binoculars help to distinguish them from lower cloud because NLCs have sharp well-defined wave structures.