This occurrence is as rare as it is beautiful, as is often the case with rainbows. Just weeks ago, residents of Tokyo, Japan filmed an incredible sight that soon went viral on social media.A "rainbow cloud"Scientists call it a "circumhorizontal arc" but this optical phenomenon is better known as "rainbow cloud" or "rainbow of fire," because it's sort of shaped like a flame.Tokyo residents who spotted it excitedly rushed outside to get a better look at it. Here's a video of the cloud that graced Japanese skies on June 26, 2020. It requires very specific conditions, which makes it a rare occurrenceSimply put, this "rainbow" can be seen when the sunlight is reflected by the ice crystals that make up the cloud. In this case, this cloud is a cirrus cloud, somewhere between 5,000 and 14,000 meters above sea level, that is made up of such ice crystals.These crystals have to have a specific shape and receive light from a specific angle... In short, it takes a complex set of conditions for this to happen. In fact, it could never happen in certain parts of the world. The farther away you are from the equator, the less likely you are to observe this phenomenon.That's why it can only be seen a few times a year in France, although you're much more likely to spot it in the south. It's more common in the United States than in Europe, for example. Conversely, this rainbow of fire has little to no chance of showing up in Iceland or Scandinavia.