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The information discussed in the CleanTechnica article Full Fledged 5 Alarm Climate Emergency In Antarctica is certainly disconcerting, but the stock photo of the sky and various atmospheric phenomena of sunlight scattered in the atmosphere is still beautiful. My question is about this photo.

I see two circles around the sun, meaning light is scattered at certain angles ABC where B is the scatterer, A is the Sun, and C is the observer. However there are several features that are oriented with respect to the the local horizontal and vertical as well. For example, a long line extending to the left and right from the Sun parallel to the horizon with bright spots where it intersects the inner ring, and the beginnings of arcs, tangent to the tops of the circles but curving vertically upward.

Question: Why are these patterns of scattered sunlight aligned to the local horizontal and vertical directions?

Scattered sunlight pattern on a cold day

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  • $\begingroup$ it is called bisol and is created by flat icecrystals in the air.one have to be at the right place at the right time to see it. $\endgroup$ – trond hansen Jun 18 '18 at 8:08
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    $\begingroup$ Possible duplicate of Are there measurements or calculations that suggest atmospheric ice plates would be horizontal to within 0.1 degrees? It turns out that this was already answered at another question I asked a year and a half ago, related to things seen from space! $\endgroup$ – uhoh Dec 17 '18 at 5:28
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    $\begingroup$ good catch,i have seen it in real life and will never forget how beautiful it was. $\endgroup$ – trond hansen Dec 17 '18 at 6:47
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    $\begingroup$ @trondhansen very nice! I've seen the sun dogs a few times and the inner ring once around 45N but now I'm too close to the equator to see these very often. Instead, during the summer I can see my shadow disappear underneath me at noon. $\endgroup$ – uhoh Dec 17 '18 at 7:12
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    $\begingroup$ It looks like a solar ice halo $\endgroup$ – Fred Dec 21 '18 at 13:53
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Note: All of the links in this answer point to various parts of Les Cowley's excellent Atmospheric Optics site. Les combines photographs, simulated images, and descriptions of the underlying physics to beautifully portray and explain a number of different optical effects in our atmosphere.

Note: The imagery on that site is copyrighted. Please do not edit this answer to portray images from that site.


That's a very nice picture that shows a number of ice halos. Ice crystal halos are caused by various shapes of ice crystals. Some of these halos such as the small circle that encircles the Sun in the image in the question are easily seen at all latitudes. Others only appear in very high latitudes, and even there they are rather infrequent.

The small circle is the 22° halo. This is a frequently seen halo, much more common than rainbows. It's caused by poorly aligned hexagonal prism ice crystals. If you look closely, you'll see that the sky is darker inside the 22° halo. This is similar to how the sky is darker outside the primary bow of a rainbow. Unlike a rainbow, the color separation is very small, making the 22° halo white rather than colored. If you look closely, you can see that the inner edge has a bit of a reddish tinge.

The two bright spots on the sides of the 22° halo are sun dogs. These result from hexagonal plates of ice with their large faces nearly horizontal. The curve the intersects the Sun, runs parallel to the horizon, and intersects the 22° halo at the sun dogs is the parhelic circle. Parhelic circles result from sunlight reflecting off nearly vertical faces of ice crystals. The mustache atop the 22° halo is the upper tangent arc. Tangent arcs result from ice crystals that form hexagonal columns rather than hexagonal plates. As is the case with the 22° halo, sun dogs, parhelic circles, and tangent arcs are fairly common.

The partial circle outside the 22° halo is the not so common supralateral arc, or possibly the even rarer 46° halo. Atop that is the circumzenithal arc.

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    $\begingroup$ At night you can see similar, but dimmer effects from a bright moon. If you know your stars well, you can also see 'extra' stars from the sundogs from bright stars. These are called 'star pups' I've not seen them. To notice them you would have to really know your night sky. $\endgroup$ – Sherwood Botsford Jun 18 '18 at 12:12
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Why are these patterns of scattered sunlight aligned to the local horizontal and vertical directions?

The answer can be found in The New York Times article Light Pillars, a Million-Mirror Optical Illusion on Winter Nights and in The Washington Post's Rare light pillars cast colorful columns into Wisconsin’s night sky says:

However, that’s just half the story. The formation of light pillars requires hexagonally shaped ice crystals in the air to reflect the light. Moreover, they have to be oriented horizontally, like flying discs or plates to bounce light back toward the observer. Less-organized orientations can produce sun pillars and halos.

A more physics-based explanation can be found in this answer to the question Are there measurements or calculations that suggest atmospheric ice plates would be horizontal to within 0.1 degrees?

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Also from this tweet:

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