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.

How do birds get their color? Fantastical implications for designing new life forms.

One of the fantasies I like to entertain is that it may someday be possible for humans to create a whole new biota of life. I know, I know. I’m a scientist. Life evolves. It is not created. But just imagine how exciting it would be for humans to create their own living offspring. I don’t know how, when, and where, but isn’t it exhilarating to think that maybe we could create offspring of our own design? Our biologists are already attempting to make primitive living structures. Maybe someday we will colonize space. Could our artificial intelligence support our new life forms? A simulator? A novel universe among multi-verses? I think the possibilities are limitless no matter how many eons into the future it may take humankind to achieve such a goal.

Neo Rauch, Self-Portrait, 1987

So, one of my artistic activities is to study Earth’s biota and imagine new organisms. I study, make juxtapositions, and create new forms and biologies.

I have a doctorate in the physiological sciences. So, I have a pretty good understanding of molecular biology and physiology. Right now I am more interested in conceptualizing new structural and external features.

As I consider novel external features, I am obsessed with color. “Color is very important to me,” I recall my grandmother saying once. Color has also become very important to me. Color has played a role in my psychiatric disorder from the beginning. At first, it seemed to me that primary colors were being placed in my environment in purposeful patterns. Then these primary colors seemed cleaner and brighter. Change in color perception is still one of the predominant symptoms I experience when my mental status is altered. So, yes, color is very important to me.

As I imagine new structures for new life forms, one of the first things I think about is color. I’ve already started imagining greenish humanoids, but then I feel unoriginal. Doesn’t everyone imagine green aliens?

What about the possibility of multi-colored humanoids? This has led me to an interest in bird coloring.

It turns out that birds derive their colors from two types of pigments: melanins which produce black, grey, brown and orange colors, and carotenoids, which are used by specialized feather structures to generate the brighter colors.

The melanins are synthesized by melanocytes and the birds’ natural physiology. Natural structural variations in feather follicles give fine control to the pigmentation. The carotenoids, which give the birds the brighter colors, must be obtained from the birds’ food, and the melanins and carotenoids work together to give the birds their intricate, colorful plumage patterns.

So, as I continue my fantasy of designing a novel biota, and when I turn to the subject of a novel organism’s color, I am now given the idea to consider interactions between naturally produced pigments, pigments in the organism’s diet, and structural contributions to pattern and pigment.

Maybe someday I will share images of my creations. Right now, the biologies, forms, and colors exist predominately in my dreams. The project has just begun.

Finding The Mother Tree

Finding the Mother Tree

By Suzanne Simard, Narrated by Suzanne Simard

About halfway through the audio version of this memoir narrated by the renowned forestry scientist Suzanne Simard herself, I was ready to conclude that the author had not accomplished her goal of getting me to see the forest in anthropomorphic terms.  Now that I have concluded the book, this is in some ways still true for me, but by the end of the book I was able to see the parallels between Dr. Simard’s personal story and the life of the forest.  I eventually stepped into the shoes of both Dr. Simard and the forest.

This is a story of innate joy in curiosity turned into disciplined science and academic activism.  It is a story of communication, competition, cooperation, and synergy.  

In this book, you will learn how the forest works as a united whole, and you will find broad understanding and implications for human life with friends, family, and society in general.  By the end of the book, the title truly has a specially developed meaning.  You will find the mother tree.

Note:  I found the audiobook could be comfortably enjoyed at 1.5X.