Strange Sights

Today was a beautiful day. The sun was shining, but it wasn't too hot. White clouds were skating across the blue sky. I was enjoying the touches of yellow and orange that are starting to dot the hillside. And suddenly I was distracted by this sight:


At first, I thought it was caused by the polarized lenses in my sunglasses... But it was still visible when I took them off. In reality, it was even more brightly colored than this photo shows. The lavender was much more vivid, and the bright blue that streaks from left to right was more obvious. The cloud on the right looked like flames rising as the colors morphed from one to another. It reminds me of the aurora borealis. Anyone know what would cause this beautiful sight?