It's one of the signs of summer -- thin, glowing clouds that appear high in the sky in the hours after sunset for spots in the higher latitudes.
According to the very cool website Spaceweather.com:
"Every summer since the late 19th century, Earth's polar skies have lit up with gossamer, electric-blue clouds, twisting and rippling in the twilight sky. They're called noctilucent ('night-shining') because they can be seen after dark. The origin of the clouds, which hover at the very top of Earth's atmosphere in close proximity to space itself, is uncertain. They have been linked to causes as varied as meteoroids, climate change, and the icy exhaust of the space shuttle."
Here is some time lapse video taken from Norway on Wednesday morning. Aside from watching the glowing clouds form and drift, watch the stars march across the sky. Also of note: it's in the initial stages of "White Nights" as you still see some sunlight through the night, and how the sky begins to get brighter around 3 a.m:
While they are more common in the far northern latitudes, we can see them around here sometimes. Here is a photo of the clouds taken by Reid Wolcott last summer: