Did you know that there is something called a reverse rainbow?

After heavy rains, you suddenly look up in the sky and see a band of colourful arcs spreading across the sky. These arcs, what we know as rainbows, look surreal and magical, no matter how many times you see them. They want to make you believe in magic every time you see them, don’t they? We are all used to the classic rainbow. But did you know there is a phenomenon called reverse rainbow? Yes, they exist and are also known as upside-down rainbow! Actually, reverse rainbows are not really rainbows, they are circumzenithal arcs. Circumzenithal arcs are like the halos you see around the sun or moon. Circumzenithal arcs are the most beautiful of all halos. You might be familiar with the process of rainbow formation, right? Let's find out how the reverse rainbow is formed and much more. 

Circumzenithal arcs: How are they formed?  
Circumzenithal arcs are formed due to the presence of ice crystals in the upper crust of the atmosphere. Light from the sun is refracted by ice crystals. It enters the crystals from the top surface and leaves through the side face, leading to an upside rainbow effect. The observer's position also plays a vital role in determining the visibility of circumzenithal arcs. So, if you ever witness a circumzenithal arc, know it's truly a magical coincidence! 

The colours of circumzenithal arcs 
Now you know what a circumzenithal arc is and how it is formed. They may look like rainbows, but they are quite not. This is because the colours in a rainbow are aligned in a familiar pattern, which people call VIBGYOR in short, with the red end of the spectrum closer to the sky and the violet end closer to Earth. In the case of circumzenithal arcs, the violet colour is closer to the sky and the red band is closer to the planet.  

How common are they? 
You may think of circumzenithal arcs as rare, but they can be fairly common. A particular set of conditions is required for the formation of a circumzenithal arc. The sun should be more than five and less than 32 degrees above the horizon. This is needed so that sunlight can refract correctly to form the smiling halo. Also, the clouds should be one with ice crystals, known as ‘cirrus clouds.’ These clouds occur throughout the year, making the circumzenithal arcs quite common. People only see them sometimes, as most of the time, circumzenithal arcs are hidden by clouds. So, you  need a clear sky to witness reverse rainbows.