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.
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.
Circumhorizontal arcs can be wide bands or narrow. The orientation of the colors of the arc always remains parallel to the horizon.
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?
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?
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°.
Circumzenithal arcs are also known as “upside down rainbows”.
Watch: Circumhorizontal arcs
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/
