Environment
Fact-checked

At AllThingsNature, we're committed to delivering accurate, trustworthy information. Our expert-authored content is rigorously fact-checked and sourced from credible authorities. Discover how we uphold the highest standards in providing you with reliable knowledge.

Learn more...

What is a Circumzenithal Arc?

Mary McMahon
Mary McMahon
Mary McMahon
Mary McMahon

A circumzenithal arc is a fascinating atmospheric phenomenon, sometimes called a reverse rainbow, because at first glance it does indeed resemble a backward or upside-down rainbow. Many observers miss out on circumzenithal arcs, because they are located directly overhead; astronomers cite the circumzenithal arc as another reason for people to look up more, as if the stars weren't enough. Unlike a rainbow, which appears opposite the sun, a circumzenithal arc is centered around the zenith of the sky, and can only appear if the solar angle is less than 32 degrees.

In order for conditions to be right for a circumzenithal arc to form, small, flat, six sided ice crystals must be suspended high in the sky to create a field of tiny prisms. The sun's rays enter the ice crystals and reflect through them, projecting an arc in the sky which, if complete, would circle the zenith. Completely circular circumzenithal arcs are rare, however; most of them only take up a section of the sky, looking like a smile looking down from the heavens. The circumzenithal arc will remain until the solar angle changes, unless weather conditions change dramatically.

Man holding a globe
Man holding a globe

The colors of a circumzenithal arc are also reversed from those of a rainbow; the violet end of the spectrum is closer to the zenith of the sky, while the red range is closest to Earth. Technically, the red range is still at the top of the arc, but because the arc is reversed, the colors seem upside down as well. When looking at a circumzenithal arc, observers may notice sun dogs, another atmospheric phenomenon, as well, as the conditions that prevail to permit the formation of circumzenithal arcs are also optimal for sun dogs.

In most cases, a circumzenithal arc will last at least half an hour, and sometimes more, plenty of time to admire and photograph the beautiful arc. They are most common in colder climates, where ice crystals tend to collect in the sky with abundance, although they can be seen in temperate zones as well, especially during cooler weather. Just like with a rainbow, it is also possible to see a circumzenithal arc from the inside of an airplane, although it may be difficult to distinguish from a rainbow due to the unique angle the observer is seeing it from.

Frequently Asked Questions

What is a circumzenithal arc?

A circumzenithal arc is a stunning atmospheric phenomenon that appears as a brightly colored, upside-down rainbow high in the sky. It forms when sunlight refracts through ice crystals in cirrus clouds, bending the light into an arc that spans a portion of the circumference around the zenith, which is the point directly overhead.

How often do circumzenithal arcs occur?

Circumzenithal arcs are relatively common but often go unnoticed due to their position high in the sky. Their visibility depends on the presence of the right type of clouds and the angle of the sun. They are more frequent in colder climates where ice crystals in the atmosphere are more prevalent.

What conditions are necessary for a circumzenithal arc to form?

For a circumzenithal arc to form, the sky must have cirrus clouds containing hexagonal ice crystals. The sun needs to be at a low angle, typically less than 32 degrees above the horizon. The ice crystals must be oriented horizontally to refract the sunlight into the characteristic spectrum of colors.

Can circumzenithal arcs be seen anywhere in the world?

Circumzenithal arcs can be observed anywhere in the world, provided the atmospheric conditions are right. However, they are more commonly seen in higher latitude regions due to the frequent presence of ice crystals in the atmosphere. They are less common in tropical regions where the necessary ice crystals are rarer.

Is a circumzenithal arc the same as a rainbow?

While both are optical phenomena involving light refraction, a circumzenithal arc is not the same as a rainbow. Rainbows are caused by light refracting through raindrops, while circumzenithal arcs occur when light passes through ice crystals. Additionally, circumzenithal arcs are much rarer and appear closer to the zenith rather than the horizon.

What is the best time of day to see a circumzenithal arc?

The best time to see a circumzenithal arc is when the sun is low in the sky, typically no more than 32 degrees above the horizon. This often occurs during late fall and winter months in the mid-morning or late afternoon. The specific timing can vary based on your geographic location and the time of year.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Learn more...

Discuss this Article

Post your comments
Login:
Forgot password?
Register:
    • Man holding a globe
      Man holding a globe