Years ago,literally the very week I moved to Colorado, I saw a truly weird thing out my window. It was a meteorological phenomenon (or "cloud thingy" as nebulaphiles call them) that I had never seen before, and it really threw me at first.
What I saw was a cloudbank that stretched from the east to just before the mountains. There,it stopped in a very nearly straight line that reached north and south as far as I could see. To the west of it was absolutely clear blue sky.
Source: blastr.com