Days with high thin cirrus clouds are also days when jet contrails develop, and that can make for some cool optical phenomena in the sky!
Take this picture for instance: a sundog (parhelion) shows up directly left of the sun in a cirrus cloud formation called a “mare’s tail.” The little bright spot that looks like a small rainbow wouldn’t be there if not for that wispy cloud.
Irina Bodie sent this one to us; it’s a small part of a 22º halo that is visible only because of the wind-swept contrail. The clear air around it doesn’t refract the sunlight like the ice crystals in the cloud, so that’s why you only see a tiny part of it. The contrail started as jet exhaust; ice crystalized around the small particles in the exhaust plume and took the form of a cirrus cloud:
Michelle Miklik caught the whole picture of the 22º halo, sundogs, and a tangental arc (the brighter rainbow at the top of the circle) in Calhoun County:
And then there were some awesome sunsets like this!