Noctilucent clouds (NLCs) in the dead of night, over Hedmark county, SE Norway, at 60.8șN latitude.
Time 17.07.2008 01:18 (GMT +1 / Daylight savings +1). In other words, exactly when the sun is at its lowest under the horizon,
the very darkest moment of the night. Note how intensively the northern sky illuminates, due to the silvery and shimmering NLCs.

This phenomenon is rare, and shows the highest clouds existing on this planet,
hovering at altitudes from 80-85km (50-53 miles) above the earth,
close to the mesopause, the coldest part of the atmosphere.
Normal and darker tropospheric clouds at a mere 1-5 km height are here present beneath the NLCs.
A closer explanation of the physics can be found here:
Atoptics.co.uk - Noctilucent clouds
Atoptics.co.uk - About NCLs