The fire rainbow is the rarest of all naturally occurring phenomena. The clouds must be cirrus and at an altitude of 20,000 feet at least. There must be just the right amount of ice crystals present, as well. The sun has to hit the clouds at exactly 58 degrees. It makes the rainbow appear to be on fire, hence the name. It’s actually cold as ice, though. In the weather world, the phenomena is known as a circumhorizontal arc.