Have you ever seen a faint arc of colours in the sky? This optical phenomenon is known as a circumhorizontal arc and is often mistaken for a rainbow. However, it's much rarer and can only occur under specific conditions.

What is a Circumhorizontal Arc?

A circumhorizontal arc is a rare optical phenomenon that occurs when sunlight passes through ice crystals in the atmosphere. These ice crystals are usually found in cirrus clouds, which are thin and wispy clouds that form at high altitudes.

When the sunlight passes through these ice crystals, it is refracted or bent at a specific angle. This causes the light to separate into its component colours, creating a spectrum of colours that form a faint arc in the sky.

How Does a Circumhorizontal Arc Form?

A circumhorizontal arc can only form under specific conditions. Firstly, the sun needs to be at an angle of at least 58 degrees above the horizon. Secondly, cirrus clouds need to be present, and they need to be aligned horizontally.

When the sun is at the correct angle, the sunlight passes through the ice crystals in the cirrus clouds at a specific angle. This causes the light to refract and separate into its component colours, creating the arc.

Where Can You See a Circumhorizontal Arc?

A circumhorizontal arc can be seen anywhere in the world as long as the conditions are right. However, they are more common in areas that have a high concentration of cirrus clouds, such as the polar regions.

If you do see a circumhorizontal arc, it will usually be visible for around 20 minutes or so. However, the appearance of the arc can vary depending on the location and the angle of the sun.

Conclusion

A circumhorizontal arc is a rare and beautiful optical phenomenon that is caused by sunlight passing through ice crystals in cirrus clouds. It's a stunning display of nature's beauty and is definitely worth keeping an eye out for.

So next time you're outside on a sunny day, take a moment to look up at the sky. You never know, you might just spot a faint arc of colours.

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