They appear as electric blue wisps well after sunset - far too long to be normal clouds, but there they are reflecting sunlight from about fifty miles above the Earth. I've only been lucky enough to see two noctilucent clouds in my life so far and I've been hunting since I was a teenager.
Fifty miles up the atmosphere is nearly a vacuum - a very dry vacuum. Ice crystals can form if it is cold enough. During the Summer the region chills to about -120°C. A bit of dust - most of it is thought to be fine particles - a smoke - from micrometeorites act as nucleation seeds to start the formation of ice crystals. When the Sun is low enough to catch them leaving normally brighter clouds and the sky glow dark they can pop into view.
They've only been noticed since 1885 - a few years after the eruption of Krakatoa. People were in the habit of watching the beautiful sunsets produced by volcanic dust and reports of these odd wispy clouds that were too high up to be clouds started to appear. Since then sighting have become more common and their frequency seems to be increasing. A few years ago some were spotted as far South as 35°. I've seen them at 40° and 53°.
There are still a large number of questions and they may be gauges of other activities. One hypothesis that is gaining strength is their strength is related to the amount of methane in the upper atmosphere. Their frequency and strength may be an indicator of natural gas that has escaped from wells and leaking pipelines.
If you're in Canada, most of Europe - even the Northern US - go out and look at the sky around an hour after sunset on clear nights. Seeing them once is worth looking hundreds of times - they're very special. If you're good with a camera and low light situations these are beautiful subjects.
maybe, just maybe ...
the beautiful photo was taken in Stockholm by Kevin Cho last year
Every once and awhile it is good to list something to avoid. I grew up in a region where the dessert salad ruled. They seem to be associated with the upper midwest and Scandinavian populations, but their ease of preparation and women's magazines probably caused them to spread in the 50s and 60s. Concoctions like glorified rice, ambrosia, seafoam salad... I was thinking about listing Ambrosia, but my mother liked Snickers Salad - just the thing to inflict on a church social when you aren't doing the hot dish.
° chop an apple (something like a Granny Smith), a Snickers bar into a bowl of Cool Whip (or some other whipped cream substitute)
° mix in some sliced banana and chopped walnuts or pecans to customize