Here's a little something that was new to me when I read it: Noctilucent Clouds.
What the heck are those? That's actually a pretty good question. They are clouds that develop during the summer months over the poles and are the highest elevation clouds that this world has. They develop in the mesosphere, a whole lot higher than most clouds develop. There isn't a lot known about them, other than they appear enhanced thanks to things like volcanic eruptions and shuttle launches. The Wikipedia entry for the phenomonon has some more info as well as some pretty pictures.