Fairbanks rainbow: usual or unusual?

[RJN]

APOD received the following image and attached email:

The emailer went on to say:

I was in North Pole Alaska (near Fairbanks) on July 5, 2011 and saw this unusual rainbow. It was wavy and had spaces of no color between large sections. I wondered what caused this phenomena?

I saw the rainbow with my eyes, it was so unusual that made me decide to take the pictures.

This was about 7 PM on July 5th. There was no rain. It was a very warm day. The Sun was at my back (yet still overhead as we were so far north, I think they said we were 500 miles south of the north pole). We were facing southeast, I believe. The portions of the rainbow were apart when we first looked, then came together.

So my question to alert APOD readers is: Is this just a typical rainbow, or is something unusual going on here? Your thoughts would be appreciated!

- RJN

[monam]

Almost looks like Aurora during the day?

[bystander]

Looks more like iridescent clouds.

APOD: An Iridescent Cloud Over Colorado (2007 Nov 25)
Atmospheric Optics: Corona to Iridescence
Atmospheric Optics: Iridescent Clouds

[Chris Peterson]

I'd say this is unusual, but not extremely rare. Since there is no evidence of rain, I think this would better be called a cloud bow, or more precisely in this case, a spoked cloud bow. What we are seeing is probably a rainbow produced in the very small droplets of the thin cloud layer, broken up by anticrepuscular rays.

(Although the Sun is described as "overhead", it was actually at an altitude of about 25° when the image was made, which is consistent with the altitude of the cloud bow. The peak of the bow would have been almost due east, so it makes sense that this image was made towards the southeast.)

[neufer]

June 19, 2006—It looks like a rainbow that's been set on fire, but this phenomenon is as cold as ice.

---

<<Known in the weather world as a circumhorizontal arc, this rare sight was caught on film on June 3 as it hung over northern Idaho near the Washington State border.

The arc isn't a rainbow in the traditional sense—it is caused by light passing through wispy, high-altitude cirrus clouds. The sight occurs only when the sun is very high in the sky (more than 58° above the horizon). What's more, the hexagonal ice crystals that make up cirrus clouds must be shaped like thick plates with their faces parallel to the ground.

When light enters through a vertical side face of such an ice crystal and leaves from the bottom face, it refracts, or bends, in the same way that light passes through a prism. If a cirrus's crystals are aligned just right, the whole cloud lights up in a spectrum of colors.

This particular arc spanned several hundred square miles of sky and lasted for about an hour, according to the London Daily Mail.>>

<<Look for the brightly coloured circumhorizon arc (also a circumhorizontal arc but never 'fire rainbow') when the sun is very high in the sky - higher than 58°. Near to noon in mid summer is a good time in middle latitudes. The halo is beneath the sun and twice as far from it (two hand spans) as the 22º halo.

It is a very large halo and always parallel to the horizon. Often only fragments are visible where there happen to be cirrus clouds - the individual patches of cirrus are then lit with colour that can be mistaken for iridescence.

High sun infralateral arcs occupy a similar position in the sky. The infralateral arc curves upwards from the horizon slightly and this is best checked for visually because lens distortions invariably also show the circumhorizon arc as curved.

The rarity, or otherwise, of the arc depends on where you are. At medium latitudes like much of the USA it is not rare - it can be seen several times each summer. In contrast, further north in much of Europe the circumhorizon arc is a rarity and impossible to see north of Copenhagen. See the charts in 'How rare?' for the visibility at your location.>>

[Chris Peterson]

<<Look for the brightly coloured circumhorizon arc (also a circumhorizontal arc but never 'fire rainbow') when the sun is very high in the sky - higher than 58°. Near to noon in mid summer is a good time in middle latitudes. The halo is beneath the sun and twice as far from it (two hand spans) as the 22º halo.

The Sun never exceeds an altitude of 48° in North Pole, Alaska, and was only at 25° when this image was made.

[neufer]

<<Look for the brightly coloured circumhorizon arc (also a circumhorizontal arc but never 'fire rainbow') when the sun is very high in the sky - higher than 58°. Near to noon in mid summer is a good time in middle latitudes. The halo is beneath the sun and twice as far from it (two hand spans) as the 22º halo.

The Sun never exceeds an altitude of 48° in North Pole, Alaska, and was only at 25° when this image was made.

Indeed, and I was just about to delete the post when you foiled my attempt. (curses!)

[owlice]

I think it's a cloud bow, too, or possibly a rainbow if it's sprinkling off in the distance. First, it is generally bow-shaped, so rules out a circumhorizontal arc, as it's not parallel to the horizon. It appears away from the sun -- looking closely at the picture, it appears the sun is to the right of the photographer (given where the shadows are on the fence posts), so I think too far from the sun to be iridescence, which usually appears closer to the sun, not in the antisolar direction. There is very little shadow showing, too, so that indicates that though the sun is off to the right, it's also behind the photographer. That the phenomenon is in the antisolar direction is also a point in favor of a bow.

[Guest]

I think there was something between the sun and the water particles
that scattered the light and prevented the light rays to disperse at
the water particles. Looks like it was a cloud.

Here two examples for scattered sun light that is partly blocked by cloud parts:

http://static.panoramio.com/photos/orig ... 512657.jpg
http://static.panoramio.com/photos/orig ... 260637.jpg

It could also be winds that scattered the fog.

[AnudderGuest]

Couldn't it be a normal rainbow except parts of the bow are being "prevented" by the shadows of higher clouds causing the anticrepuscular rays? Perhaps this is what the first poster was saying also. Regardless, I just think it's pretty

[Chris Peterson]

Couldn't it be a normal rainbow except parts of the bow are being "prevented" by the shadows of higher clouds causing the anticrepuscular rays? Perhaps this is what the first poster was saying also. Regardless, I just think it's pretty :-)

I think that is basically what it is. But an actual rainbow is produced by rain, and the sky conditions here don't suggest rain, which is why I think it is actually a cloud bow, which is closely related. Also, the color saturation and brightness of a rainbow is associated with droplet size. The low saturation of this bow is a further indication that it is formed in clouds (small droplets), not rain.

A rainbow that is broken in pieces by anticrepuscular shadows is called a spoked rainbow, which is why I suggested spoked cloud bow for this phenomenon.

[Harry TenHerons]

Anticrepuscular ! A Wonderful Word !