Once in a while one can see unusual things in the sky. When this photo was taken at approx. 23 hr the sky should have been dark blue but it wasn't.
Noctilucent clouds are the highest clouds in the earth's atmosphere: they occur between 75 and 85 km altitude. Normally they are too faint to be seen. There's a nice article about this phenomenon at wikipedia: http://en.wikipedia.org/wiki/Noctilucent_cloud

What I find interesting is that there are at least three (four?) layers of clouds visible at the same time, each with specific and different shapes. The high contrast and colours of the sky add to the visual interest.