When we were in Oslo between Christmas and New Years, we saw some very strange looking clouds. We were in Frognerparken, a well-known park with hundreds of statues by the same sculptor, Gustav Vigeland. The sun had already set, but these clouds, rather than turning red or dark gray, were a brilliant white. I guessed they were so high up in the atmosphere that the sun shone on them from below the horizon without going through a lot of air, which would turn the light red.
Only later did I learn that they were called nacreous clouds, or Polar Stratospheric Clouds. I was right about why they glowed white, but I didn’t know that they were quite rare in places humans live. To form these clouds, temperature must be very low. That means in polar winters. My friend Joe had seen them in Antarctica.
It had been very stormy just before that, so it’s possible that the nacreous clouds were created by something call “lee waves,” which can push water vapor to an altitude where they don’t belong. Here is an article from a Norwegian newspaper about this.