Noctilucent clouds form near the top of Earth’s polar atmosphere when water vapor from the planet below mixes with meteor debris from space. They appear during summer because that is when the mesosphere is coldest and most humid. This year, they appeared early, more than a full month before the solstice, setting the stage for an unusually good NLC-watching season.
High-latitude watchers should be alert for repeat shows.
Look west 30-to-60 minutes after sunset.
And if you see blue-white tendrils?