Noctilucent clouds (NLC) or with other name polar mesospheric clouds. So far they have only seldom been sighted at our latitude, but for a week now they have sometimes been seen. This is the first time I've managed to take some photos of them. They appear between the end of May and mid-August well after sunset, when the Sun is 6-16° below the horizon. These pictures were taken between 21:55 and 22:15 in Vecsés, Hungary (the time of sunset was 20:44). Noctilucent clouds are the highest clouds, they are in the mesosphere some 85 km above us. For this reason, their formation is still a mystery, but according to the latest research it might be in connection with the water content of the exhaust of space shuttles and rockets. Satellite mission AIM is exploring their nature now.
Noctilucent Cloud song and lyrics :)