Coronas

A circle of light quite distinct from the 22° halo can sometimes be seen around the sun or moon if there are thin clouds composed of water droplets or ice crystals of nearly uniform size. This circle is smaller, on the order of 10°, and is caused by diffraction of light by the small particles. The appearance is often that of alternating blue-green and red circles. The diffraction by a tiny opaque barrier shows the same pattern as that of an aperture of the same size and shape in an otherwise opaque screen (Babinet's principle).

The moon corona image above was taken on March 19, 2005 on a night when thin clouds were moving swiftly across the sky. The colors are subtle, but you can see a blue tint to the corona close to the moon and a reddening further out. This indicates diffraction in tiny water droplets. The shorter wavelength blue light produces diffraction maxima closer to the sun.

Corona from fogged eyeglasses
What size droplet would it take to diffract light at 22°?
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Glories

A rainbow band around the shadow of your aircraft seen on a cloud below is an example of a glory. For more distant clouds, the plane's shadowwould be smaller, but the glory would be the same angular size if the waterdroplets were the same size.

The phenomenon is one of diffraction, with smaller droplets givinglarger glories, andMie scattering isthought to be themechanism controlling theintensity of lightscattered back tothe viewer. However,mathematical modeling of the details is quite complex.
Glory on a nearby cloud
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Glory on Close Cloud

This glory was observed on an approach to Anchorage through clouds which were sometimes very close, sometimes distant. At this point the aircraft shadow is larger than the glory, but as the clouds became more distant, the shadow was smaller while the glory stayed the same size. Remarkable telescoping of the shadow size occurred within the glory.

Examples of glories on close and distant clouds
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Iridescent Clouds

The corona around the sun is often hard to see because it is close to the sun and masked by the extreme brilliance of the sun. When the same diffraction effects occur in clouds some distance from the sun which are composed of tiny uniform droplets, colorful bands or "iridescence" can be seen.

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Heiligenschein

The heiligenschein or "holy light" is like the glory except that it is seen as a white glow around the shadow of your head projected on fog or mist. It can also sometimes be seen around your shadow on a rough or dusty surface (in this case called the "dry heiligenschein"). The area immediately around your head's shadow may be seen as slightly brightened by "head on" incidence of light on small particles while at slightly greater distance from the head's shadow you get darkening because you begin to see the shadows of the small particles which are now being lit at a greater angle of incidence. Dew heiligenschein can be produced by retroreflection of the light focused by tiny dewdrops, while this light from droplets further from your head's shadow does not reach your eyes.

Examining dew heiligenschein a bit more closely, following the discussion of Lynch and Livingston, tiny dewdrops act as essentially spherical lenses, focusing the light at a distance of about 0.7 x the drop radius behind the drop. If that concentrated light strikes a leaf or blade of grass, part of it is retroreflected through the droplet, proceeding backward toward our eyes. The retroreflection produces the appearance of a bright diffuse glow around the viewer's head. It is spread over a few degrees by the fact that the droplet "lenses" have large amount of aberration and don't send their light back at exactly 180° .

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Reference
Lynch and Livingston
Sec 4.12
 
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Specter of the Brocken

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