Noctilucent clouds
Electric-blue, rippled clouds shining in a dark summer sky, far higher than any weather.
WhenLate May to early August at latitudes of roughly 50–65°WhereLow in the north (or south in the southern hemisphere) one to two hours after sunset or before sunrise
Why it happens
They form about 80 km up, at the edge of space, where the air is at its coldest in summer. They are lit by a sun that has already set for everything below them, which is why they glow while ordinary clouds are dark silhouettes.
How to see it
Look north on a clear summer night from about an hour after sunset. Ordinary high cloud at that time is grey; noctilucent clouds are silver-blue with fine ripples, herringbone and whirls, and they brighten as the sky darkens. Displays can last all night in the far north.
Field note. Summer nights are short: the best window is usually between 11 pm and 2 am, and a dark northern horizon helps.