Fire Rainbows” are neither fire, nor rainbows, but are so called because of their brilliant pastel colors and flame like appearance. Technically they are known as circumhorizontal arc. Brightly colored circumhorizontal arc occur mostly during the summer and between particular latitudes. When the sun is very high in the sky, sunlight entering flat, hexagon shaped ice crystals gets split into individual colors just like in a prism. The conditions required to form a “fire rainbow”.
Fire Rainbows are so large that sometimes we see only parts of them where they happen to 'light' fragments of cirrus cloud. They are so huge that their colours sometimes appear to be those of the sky itself rather than an ice crystal halo. The sun has to be at an elevation of 58° or greater, there must be high altitude cirrus clouds with plate-shaped ice crystals, and sunlight has to enter the ice crystals at a specific angle. This is why fire rainbows is such a rare phenomenon.