Sometimes clouds take on rainbow colors - hazy greens and reds. Fire rainbows are the turbo-charged versions of that. They're rainbows that seems to be turned into wispy clouds, making them look like sky fires. Although fire rainbows are rare, the physics behind them is pretty simple.
Fire rainbows are very, very cool looking. They're a combination of clouds and rainbows, with the brilliant colors of a rainbow conforming to the contours of a cloud. Since fire rainbows are almost always found in cirrus clouds, the resulting wisps look like curls of flame moving across the sky. How do we get this crazy phenomenon?
It starts with sun on ice crystals. This is why cirrus clouds are generally the only clouds to display fire rainbows. The clouds have to be far enough up in the air for ice crystals to form, and the sun has to be shining through them in order for the light to reach people on the ground. Dark storm clouds blot out the sun and low fog doesn't contain enough ice crystals to properly refract the light. When they form, ice crystals often make little hexagonal plates. If the sun is at least fifty-eight degrees above the horizon (the optimum angle is 67 degrees), its light hits the top of these plates and is split into different colors. Since the plates only exist in the cloud, and nothing else around it, the entire cloud turns into a self-contained rainbow.
If you're too far north or too far south, you can't ever see these clouds. Likewise, there are certain times of year where they disappear. The sun doesn't get high enough in the sky to create them. Still, a few photographers have been there at just the right moment and captured gorgeous photos of them in the sky.
Top Image: Dekh
Second Image: Irargerich