Fire rainbows are the rarest of all naturally occurring atmospheric phenomena. For a fire rainbow to occur, cirrus clouds must be 20,000 feet in the air with the precise amount of ice crystals, and then the Sun must hit the clouds at 58 degrees. *Stardust*
Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form; a commonly held misconception. In fact, mammatus are usually seen after the worst of a thunderstorm has passed.