Many of our viewers shared pictures of what appeared to be a rainbow on top of thunderstorm clouds Sunday evening. What they were actually looking at is what is known as a ‘pileus iridescent cloud’. I promise, I’m not making this up! It’s actually a pretty neat thing to see, and something that doesn’t happen all too often.
The above picture shows a thunderstorm cloud – or towering cumulonimbus cloud. All thunderstorms have updrafts and downdrafts. The updraft of the storm is what brings the warm, moist air into the thunderstorm. The air that is rising will reach a point in the atmosphere where it can’t rise any further and it basically flattens out. This is what is known as the ‘anvil cloud’. Sometimes the rising air can be so strong inside that thunderstorm that it breaks through the stable layer producing what is known as an ‘overshooting top’.
In this particular storm there is what looks to be a flat, smooth cloud on top of the thunderstorm. This is known as a ‘pileus cloud’. These form when there is moisture above the thunderstorm that condenses into a cloud. Similar to how lenticular clouds form – and almost looks like one.
The small water droplets inside that cloud will then diffract the sun’s light, causing the rainbow appearance above the storm. It’s actually a pretty cool process. And while the pileus cloud isn’t all that uncommon, conditions have to be just right in order to get the cloud iridescence (rainbow)!