Sky Watchers: Perplexed by These Rainbow-Colored Clouds?

All rainbows in the clouds aren't sun dogs

Rainbow colours on ice crystals in Jet stream wind clouds over the Annapurna Himalayas in Nepal,
Ashley Cooper / Getty Images

Few sky watchers have ever mistaken a rainbow before, but rainbow-colored clouds are victims of mistaken identity every morning, noon, and twilight.

What causes rainbow colors within clouds? And which kinds of clouds can appear multi-colored? The following rainbow-colored cloud field guide will tell you what it is you're looking at and why you're seeing it.  

Iridescent Clouds

Rainbow colours on ice crystals in Jet stream wind clouds over the Annapurna Himalayas in Nepal,
Iridescent clouds have the same shimmer as oil sheen on water's surface. Ashley Cooper / Getty Images

If you've ever spotted clouds high up in the sky with colors reminiscent of the film on a soap bubble or of oil film on puddles, then you've most likely seen the fairly rare iridescent cloud.

Don't let the name fool you... an iridescent cloud isn't a cloud at all; it's simply the occurrence of colors in clouds. (In other words, any cloud type can have iridescence.) Iridescence tends to form high up in the sky near clouds, like cirrus or lenticular, that are made up of especially tiny ice crystals or water droplets. The tiny ice and water droplet sizes cause sunlight to be diffracted—it is obstructed by the droplets, is bent, and spreads out into its spectral colors. And so, you get a rainbow-like effect in the clouds.  

The colors in an iridescent cloud tend to be pastel, so you'll see pink, mint, and lavender rather than red, green, and indigo.

Sun Dogs

Parhelia in high level clouds above Ambleside
Sundogs always appear directly to the left and/or right of the Sun. Ashley Cooper / Getty Images

Sun dogs offer another opportunity to see fragments of rainbow in the sky. Like iridescent clouds, they too form whenever sunlight interacts with ice crystals—except the crystals must be larger and plate-shaped. As sunlight hits the ice crystal plates, it is refracted—it passes through the crystals, is bent, and spreads out into its spectral colors.

Since the sunlight is refracted horizontally, the sun dog always appears directly to the left or right side of the Sun. This often occurs in pairs, with one on each side of the Sun.

Because sun dog formation depends on the presence of large ice crystals in the air, you'll most likely spot them in very cold winter weather; although, they can form in any season if high and cold cirrus or cirrostratus ice-containing clouds exist.    

Circumhorizontal Arcs

A horizontal rainbow in the sky, Argentina
No, it's not a straight rainbow - it's a circumhorizontal arc!. Axel Fassio / Getty Images

Often called "fire rainbows," circumhorizontal arcs aren't clouds per se, but their occurrence in the sky does cause clouds to appear multi-colored. They look like large, brightly-colored bands that run parallel to the horizon. Part of the ice halo family, they form when sunlight (or moonlight) is refracted off of plate-shaped ice crystals in cirrus or cirrostratus clouds. (To get an arc rather than a sun dog, the Sun or Moon must be very high in the sky at elevations of 58° or greater.)  

While they may not be as ahh-inducing as the rainbow, a circumhorizontal arcs do have a one-up on their multi-colored cousins: their colors are often much more vivid.

How can you tell a circumhorizontal arc from an iridescent cloud? Pay close attention to two things: position in the sky and color arrangement. Arcs will be located far below the Sun or Moon (whereas cloud iridescence can be found anywhere in the sky), and its colors will be arranged in a horizontal band with red on top (in iridescence, the colors are more random in sequence and shape).     

Nacreous Clouds

Polar stratospheric clouds
Nacreous clouds are often spotted just before sunrise or sunset in the Arctic. DAVID HAY JONES/SCIENCE PHOTO LIBRARY / Getty Images

To see a nacreous or polar stratospheric cloud, you'll have to do more than simply look up. In fact, you'll need to travel up to the world's farthest polar regions and visit the Arctic (or Antarctica in the Southern Hemisphere).

Taking their name from their "mother of pearl"-like appearance, nacreous clouds are rare clouds that only form in the extreme cold of the polar winter, high up in Earth's stratosphere. (The stratosphere's air is so dry, clouds can only form when temperatures are extremely cold, as in -100°F cold!) Given their high altitude, these clouds actually receive sunlight from below the horizon, which they reflect to the ground at dawn and just after dusk. The sunlight within them undergoes forward-scattering towards sky watchers on the ground, making the clouds appear a bright pearly-white; while at the same time, the particles within the thin clouds diffract the sunlight and cause iridescent highlights.

But don't be fooled by their whimsy—as spectacular as nacreous clouds appear, their presence allows for the not-so-nice chemical reactions that lead to ozone depletion.