Circumhorizontal Arc: Rainbow in the Clouds

Caitlin Dempsey

Updated:

When conditions are just right, water particles in clouds can act like prisms, turning the rays of the sun into a brilliant color show.

The meteorologist term for this phenomenon is circumhorizontal arc (circumhorizon arcs) and tends to happen with altocumulus, cirrocumulus, lenticular and cirrus clouds. Similar to how rainbows and moonbows form, circumhorizontal arcs form when sun light passes small water droplets or small ice crystals in the clouds, refracting the light.

Basically, what you end up with is a “rainbow” within a cloud.

Specific conditions have to happen before circumhorizontal arcs occur. The clouds must be thin and have water droplets or ice crystals that are all about the same size.


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Semi-transparent clouds or clouds that are in the process of forming are the types of clouds most likely to refract light and produce circumhorizontal arcs.

Circumhorizontal arc

There are two key aspects to identifying circumhorizontal arcs.

First, circumhorizontal arcs run parallel to the horizon.

Second, the color pattern and order in a circumhorizontal arcs is organized as you would expect with a rainbow. Starting from the top of the fire rainbow to the bottom will be the following order of colors: red, orange, yellow, green, blue, indigo, and violet.

A rainbow cloud with blue sky beneath and wispy clouds above.
A circumhorizontal arc over the Santa Cruz Mountains in California. Photo: Caitlin Dempsey.

Circumhorizontal arcs can be wide bands or narrow. The orientation of the colors of the arc always remains parallel to the horizon.

A narrow circumhorizontal arc in the blue sky. A part of a fir tree is in the lower right corner.
A narrow circumhorizontal arc. Photo: Caitlin Dempsey.

Fire Rainbow and Rainbow Clouds

Circumhorizontal arcs are also known informally as fire rainbows as these are colorful formations that, due to the wispy nature of some thin clouds, can sometimes look like fire in the sky. Another informal term is rainbow clouds.

What is the difference between circumhorizontal arcs and iridescent clouds?

A rainbow in a cloud.
An iridescent cloud. Photo: © thawornnurak /stock.adobe.com.

While both types of clouds have brilliant displays of colors, there are differences between circumhorizontal arcs and iridescent clouds.

Unlike circumhorizontal arcs which have a band of colors in the pattern of a rainbow, iridescent clouds have a random pattern of colors.

Unlike circumhorizontal arcs which are located below the sun in the sky, iridescent clouds are frequently found near the Sun.

Where in the World do Circumhorizontal Arcs Happen?

Map with countries in gray and the ocean in blue with two red lines - one for the 50 degrees north and one for 50 degrees south.
Circumhorizontal arcs are not seen in latitudes north of 50 degrees North and south of 50 degrees South. Map: Caitlin Dempsey.

According to NASA, circumhorizontal arcs are only visible cirrus clouds are present and the Sun is at least 58 degrees high in the sky.

The circumhorizontal arc cannot be observed north of latitude 55° North and south of of latitude 55° South since the Sun is always lower than 58°.

For example, London, England, which is at 51.5072° N is north of the 50 degree line of latitude and residents in this capital city would not observe rainbow clouds.

Circumzenithal arc

The refraction of sunlight through the ice crystals of a cirrus fibratus cloud are known as circumzenithal arcs. This halo occurs only when the elevation of the Sun is less than 32°.

A rainbow cloud forming an arc in the sky above wispy clouds.
Circumzenithal arc. Photo © Martin / stock.adobe.com.

Circumzenithal arcs are also known as “upside down rainbows”.

Watch: Circumhorizontal arcs

Several halos in the sky at the South Pole Station in Antarctica.
Several halos can be seen in this photo taken at the South Pole Station in Antarctica. From top to bottom: Circumzenithal arc, supralateral arc, Parry arc, tangentialarc, 22 degree halo, parhelic circle, and sun dogs on right and leftintersection of 22 degree halo and parhelic circle. Photo: Lieutenant Heather Moe, NOAA Corps, public domain.

References

A fire rainbow over West Virginia. (2021, 30). Science Mission Directorate | Science. https://science.nasa.gov/fire-rainbow-over-west-virginia

Circumzenithal arc. (2017). International Cloud Atlas. https://cloudatlas.wmo.int/en/circumzenithal-arc.html

Rainbow clouds. (2021, April 5). NOAA SciJinks – All About Weather. https://scijinks.gov/rainbow-clouds/

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.