The current season for noctilucent (NLCs) clouds is extraordinary, with repeated sightings as far south as New Mexico and southern California. This shot, perhaps, summarizes the season better than any other:
NLCs are Earth's highest clouds. They form every year around this time when wisps of summertime water vapor rise to the top of Earth's atmosphere. Molecules of H2O adhere to specks of meteor smoke, forming ice crystals ~50 miles high. When sunbeams hit those crystals, they glow electric-blue.
Noctilucent clouds normally gather around the Arctic Circle, never straying too far south. This year they have stunned observers at record-low latitudes, a possible side-effect of the Solar Minimum.