Corona and Cloud Iridecence

thick rainbow ring tight around the sun, partially obscured by a cloud. Michael Ellestad via AtOpt

Corona

The interference of light not only results in supernumerary rainbows but also rainbow bands around the sun or moon. This is a different corona than the one that is visible during a solar eclipse (which is the outer surface of the suns atmosphere), instead this rainbow colored corona is much larger and usually caused by a thin layer of clouds over the sun or moon. Other small particles like ice crystals and even pollen can result in coronae.Corona - AtOpt

diagram showing a thick repeating rainbow disc around the sun. It is spliced with monochrome violet and red sections. Corona Colors - AtOpt

Just like with rainbows, violet light makes the smallest rings, and is closet to the center. Red light makes the largest rings on the outside of each rainbow band. The center of a corona is pure white beyond the sun because these colors overlap so much. Near the edge of the overlap we see yellow and red because they are large enough not to be cover by other colors, extending past them. This white center is called the aureole.

Like how a wide variation in drop size diminishes the visibility or supernumerary bands, drop size variation diminishes how many rainbow bands can be seen.

Like with cloudbows and their supernumeraries, the size of the drops determines the size of the corona, small drops making large corona, and large drops making small corona.

Corona Drop Size - AtOpt

Unlike the interference that causes supernumerary bands (coming from the light reflected within a drop), the interference that forms coronae is from light scattering off of the surface of the drop. This means that the particles diffracting the light do not have to be transparent Modelling the Corona - Phillip Laven, which is why pollen can form coronae. A corona can be formed even if the particles are not spheres (like pollen), but the corona shape changes accordingly. Pollen Corona - AtOpt

tree silhouette, with the sun peaking from behind, blending into a vibrant white glow surounded by soft warm oval rings of rainbow colors. Mikko Peussa - via USA Today

Oval shape of a pollen corona

Coronas are an example of diffraction from a circular apature, eg. how light waves interact when they pass through a small circular hole. When light passes through a small hole onto a surface, it doesn't form a small point of light as expected, rather it forms a larger circle of light with a soft edge, and faint circular rings around it that fade out. Circular Aperture Diffraction - Hyperphysics

Cloud Iridecence

sky full of patchy thin clouds surrounding the sun. Patches of pastel rainbow colors glint off of the clouds. Nestor Andres Moreno

Cloud iridecence is strongly related to the corona around the sun. Most cloud iridecence is just a sun corona made by cloud water drops, but it lacks the regular circular form because the water droplets in the clouds are not uniform in size. There may be enough similar sized drops next to each other to create rainbow colors, but all around the sun the drop size varies, resulting in a wide variation of corona sizes.Cloud Iridescence - AtOpt

thin white cloud under the sun, its top taking on rainbow colored bands that become less even on its side further form the sun. Jennifer Lamet

Here we see the regular shape of a corona distorted by the cloud structure, forming cloud iridescence.

Nacreous Clouds

smooth bright wavy clouds blanketing a dark sky, shining with peach, gold, and a little cyan. P-M Hedén

Similar to iridecent clouds are the much rarer nacreous clouds which appear in smooth wavy sheets. Cloud iridecence can occur anywhere, but nacreous clouds are limited by an assortment of conditions that distinguish them:

  • occur during the winter
  • primarly seen in very northern climates
  • higher in the sky, formed by clouds made of ice crystals in the stratosphere
    • seen better at dawn and dusk because the sun still reaches high up, and they pop out against the dark sky
Nacreous or Tropospheric

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