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Noctilucent Clouds from Selsey, England imaged by Pete Lawrence (Digital Sky). ©Peter Lawrence.

The last few nights have seen bright NLCs over Europe.

The clouds are our highest. They are over 50 miles high and a few miles below the mesopause which at -90°C is the coldest part of our atmosphere.

The skeins of tiny ice crystals shine by reflected sunlight. At northern latitudes the sun is still above the horizon at their altitude. Summer is also the time when the mesopause is at its coldest. Changes in the upper atmosphere could be making the clouds appear more often.

Here it might appear that the moon is illuminating them. Not so, the clouds are so tenuous that the light of the moon is quite insufficient to make them visible.