What are the differences between rainbows, iridescent clouds, rainbow clouds, fire rainbows, and circumhorizontal arcs?

I’ve been interested in the topic of light scattering in the atmosphere for a few weeks now. I’ve had one of those moments of synchronicity where the subject keeps coming up, but so far I haven’t been careful enough to sort it out. So, today I had to take a few minutes to surf the internet and sort it out. This post is a summary of what I’ve learned so far.

I first heard of the topic of light scattering into colors in the atmosphere discussed in a lecture in one of the Wondrium courses I’ve been watching recently. I just kind of let the subtleties of the topic flow in one ear and out the other. Then a photo of light scattering in a cloud spotted in the Florida beach town where I grew up went viral on social media, and there was some disagreement over whether to call it a “fire cloud” or “iridescent cloud.” It seemed to be settled by experts that it was an iridescent cloud. Still, not having the differences completely sorted out in my mind, I forgot about it.

Then I started getting interested in how birds get their colors and the topic of iridescence came up again and I found I wasn’t sure how to define iridescence. Now I’ve fallen into a big rabbit hole of color and light scattering, and I think I’ll be stuck here a few days. First, I will try to tackle the rainbow/cloud topic.

Scattering of Light in the Atmosphere

Whereas reflection involves a change in direction of light waves when they bounce off a barrier, refraction of light in the atmosphere occurs when light waves change in direction as the pass from the air medium into and out of the water medium. Rainbows occur through refraction. In other words, rainbows occur when sunlight passes through rain droplets in the sky and is split into the colors of the rainbow.

A Rainbow

Cloud iridescence is the more general type of scattering of light that occurs in clouds, but it occurs through diffraction. Diffraction occurs when the light waves pass around an obstacle in their path. So in cloud iridescence, the sun’s light strikes small water droplets or small ice crystals and the light is scattered as it moves around those small objects in the process called diffraction. Cloud iridescence occurs rarely in cirrus clouds and often in altocumulus, cirrocumulus, and lenticular clouds.

An Iridescent Cloud

A “rainbow cloud” or “fire rainbow” is technically known as a circumhorizontal arc. This is a very specific type of an iridescent cloud that occurs only in cirrus clouds. The condition required to form a “fire rainbow” is very precise – the sun has to be at an elevation of 58° or greater, there must be high altitude cirrus clouds with plate-shaped ice crystals, and sunlight has to enter the ice crystals at a specific angle. So, a circumhorizontal arc is a very rare phenomenon.

A Fire Rainbow, Rainbow Cloud or Circumhorizontal Arc

So there you have it: the basic differences among rainbows, iridescent clouds, rainbow clouds, fire rainbows, and circumhorizontal arcs. How did I do, physicists?

All I know is that I love light and color, and I’m really enjoying learning about color as I consider my fantastical project of imagining new life forms.

I’m working on a zine about tricks nature uses to give animals color, and I think I’ll tackle the topic of iridescence a little more directly by looking at the topic of the iridescent paints I hope to use soon in making some art. Stick around and enjoy this rabbit hole of color with me over the next few posts.