Rainbow Clouds

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

Rainbow clouds tends to happen with altocumulus, cirrocumulus, lenticular and cirrus clouds. Similar to how rainbows and moonbows form, rainbow clouds 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.

This phenomenon is known as cloud iridescence and it doesn’t happen that often. Specific conditions have to happen before rainbow clouds occur. The clouds must be thin and have water droplets or ice crystals that are all about the same size.

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

Fire Rainbow

Also known as fire rainbows, these are colorful formations that, due to the wispy nature of some thin clouds, look like fire in the sky.

Circumhorizontal arc

Some fire rainbows form a circumhorizontal arc that runs parallel to the horizon.

A rainbow in a cloud.
A circumhorizontal arc. Photo: © thawornnurak /stock.adobe.com.

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 or south of latitude 55° since the Sun is always lower than 58°.

Circumzenithal arc

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

Rainbow clouds that are created by 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°.

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

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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