Hi Mike and Danny,
The photo that Danny has shared with us is an excellent example of cloud iridescence.
With all of the different terms appearing in this thread, I thought that I would do some research:
Circumhorizontal arcs (or fire rainbows) are a very rare phenomena that can only occur in cirrus cloud, as explained in the Wikipedia definition I have copied here:
"A circumhorizontal arc or circumhorizon arc (CHA), also known as a fire rainbow, is a halo or an optical phenomenon similar in appearance to a horizontal rainbow, but in contrast caused by the refraction of light through the ice crystals in cirrus clouds.
It occurs only when the sun is high in the sky, at least 58° above the horizon, and can only occur in the presence of cirrus clouds. It can thus not be observed at locations north of 55°N or south of 55°S, except occasionally at higher latitudes from mountains.[1]
To be visible the sun must be at an elevation of 57.8° (90 -32.2°) or more and if cloud conditions are right it is seen along the horizon on the same side of the sky as the sun. It reaches its maximum intensity at a sun elevation of 67.9° .
The phenomenon is quite rare because the ice crystals must be aligned horizontally to refract the high sun. The arc is formed as light rays enter the horizontally-oriented flat hexagonal crystals through a vertical side face and exit through the horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, makes the entire cirrus cloud shine like a flaming rainbow.[1][2]
A circumhorizontal arc can be confused with an infralateral arc when the sun is high in the sky; the former is however always oriented horizontally where the latter is oriented as a section of a rainbow, e.g. as an arc stretching upwards from the horizon.[2]
One particularly fine example was photographed over northwestern Idaho on June 3, 2006, and was reported in both the New Scientist[3] and the Daily Mail (the latter under the caption "flaming rainbow"). As the event was eventually featured on National Geographic News,[4] the news quickly spread over the internet."
The photo that I posted is definitely a circumhorizontal arc, as noted in the final paragraph above.
Then, Mike, your photo and Danny's photo are examples of cloud iridescence.
"Cloud iridescence is a common phenomenon in which a cloud shows vivid unusual colors or an entire spectrum at once.
Formation
They are formed from small water droplets of near uniform size. When the sun is properly positioned, mostly behind thick clouds, these thin clouds almost coherently diffract sunlight, and as a rainbow, different wavelengths are diffracted different amounts. Thus the colors hit the observer from different directions.
Similar examples of diffraction
See also:
Corona
Oil spill
Rainbow
For similar looking clouds, see also: Polar stratospheric cloud "
Then Sun Dogs are another different phenomenon associated with a halo around the sun and the appearance of extra images of the sun in the arc around the sun.
"A sun dog or sundog (scientific name parhelion, plural parhelia, for "beside the sun") is a common bright circular spot on a solar halo. It is an atmospheric optical phenomenon primarily associated with the reflection or refraction of sunlight by small ice crystals making up cirrus or cirrostratus clouds. Often, two sun dogs can be seen (one on each side of the sun) simultaneously.
Sundogs typically, but not exclusively, appear when the sun is low, e.g. at sunrise and sunset, and the atmosphere is filled with ice crystal forming cirrus clouds, but diamond dust and ice fog can also produce them. They are often bright white patches of light looking much like the sun or a comet, and occasionally are confused with those phenomena. Sometimes they exhibit a spectrum of colours, ranging from red closest to the sun to a pale bluish tail stretching away from the sun.[1] White sundogs are caused by light reflected off of atmospheric ice crystals, while colored sundogs are caused by light refracted through them. White sundogs are also thought to be caused by the light from the sun reflecting off of water on the ground and focusing the reflected light on the clouds above.
The ice crystals causing atmospheric phenomena are shaped as hexagonal prisms (ice Ih, e.g. with a hexagonal top and bottom and six rectangular sides). Some of these crystals are elongated, some are flat; the latter causing crisp and bright sundogs if evenly oriented with their hexagonal ends aligned horizontally, while the former produces other atmospheric phenomena, such as parhelic circles, 22° halos, circumzenithal arcs, upper tangent arcs, and lower tangent arcs. A mixture of various crystals with different alignments produces several of these phenomena at the same time.[1]
Variant of a parhelion at the South Pole. The sun is obscured.When sunlight passes through the sides of a flat crystal, both the angle of the sun rays and the orientation of the crystals affects the shape and colour of the sundogs. Misaligned or wobbling crystals produce colourful and elongated sundogs, while light passing through the crystal in non-optimal deviation angles (up to 50°) produces the "tail" of the sundog stretching away from the sun. As refraction is dependent on wavelength, the sundogs tend to have red inner edges while the colours farther from the sun tend to be more bluish-white as colours increasingly overlap.[1][2]
When the sun is low, the two sundogs are located on the circle of the 22° halo. As the sun rises, the sundogs slowly move along the parhelic circle away from the sun, finally to vanish as the sun reaches 61° over the horizon[1] (e.g. the sundogs move from the 22° halo to the circumscribed halo).[3]
On Earth, the first planet (counting from the sun) with significant amounts of ice crystal-carrying clouds, the pair of sundogs flanking the sun are aligned with the horizon. On other planets and moons where water and ice are less prevalent, however, various crystal structures produce different halos. On the giant gas planets—Jupiter, Saturn, Uranus and Neptune—other crystals form the clouds of ammonia, methane, and other substances that can produce halos with four or more sundogs.[4]
In remote stretches of western Texas, sundog refers colloquially to a segment of a common rainbow."
Michael and Jimmy have a couple of examples in the photo gallery section of their website.
I hope that this is not too much information overload!
Regards,
Geoff
