Noctilucent clouds

This is the best time of year to see these wispy, high altitude, clouds. They are most commonly seen in the night sky from 20 days before to about 50 days after the summer solstice. They form in the mesosphere at high altitudes and are seen in the northern night sky in June.

See https://sites.google.com/site/g3xbmqrp3/vuhf/noctilucent .