Over the weekend, noctilucent clouds (NLCs) broke free of the Arctic Circle and spread across most of Europe. The blue clouds, made of frosted meteor smoke, were photographed south of Madrid, nearly breaking the all time record for low-latitude sightings. NLCs are Earth's highest clouds. They form when summertime wisps of water vapor rise to the edge of space (~83 km high) and crystallize around disintegrated meteoroids. When you see one, you're literally seeing a cloud of frosted meteor smoke.