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Have you seen this stunning atmospheric display? Here's the science behind it

Thursday, January 14th 2021, 11:00 am - Simple and a few not-so-simple combinations of sunlight and tiny ice crystals result in these fantastical displays.

While skiing at Le Massif de Charlevoix, northeast of Quebec City, Dominique Jacques-Brissette captured an awe-inspiring display. Surrounding the Sun was a collection of bright halos, rainbow arcs and brilliant sundogs. What are these strange and wondrous phenomena, though, and what causes them to appear?

Atmospheric optic effects are relatively common and can happen at any time of the year. Rain showers, thunderstorms, and even a standard garden hose can produce a rainbow. Haloes can appear around the Sun or the Moon due to thin cirrus clouds high in the sky. However, the real 'magic' tends to happen on cold, sunny winter mornings. That's when we can behold spectacular displays like this:

Optics-Original-labelledThis panorama, compiled from Jacques-Brissette's video, labels the various atmospheric optics phenomena that were captured on the morning of January 4, 2021. Some of these are quite rare. Credit: Dominique Jacques-Brissette/Scott Sutherland

In the above image, stitched together from the video, there appear to be nine different phenomena. All told, however, Jacques-Brissette looks to have captured over a dozen different forms of atmospheric optics. Read on to see them all.

WHAT'S BEHIND 'THE MAGIC'?

There's actually nothing truly magic about what's happening here. It's all science!

These halos, arcs, and splotches of light around the Sun are a simple result of rays of sunlight shining through tiny, naturally-occurring ice crystals — sometimes called 'diamond dust' — floating high up in the air.

As the sunlight passes through these transparent hexagonal crystals, the light is refracted and reflected off their surfaces. This bends and scatters the light. Depending on the shape of the crystal — flat plates or long columns — and precisely which faces the light rays enter and leave the crystal by, different phenomena show up in the sky.

The most common of these phenomena are the 22° Halo, which surrounds the Sun, and the associated Parhelia, also known as Sun Dogs.

Optics-22-Halo-Sundogs-w-crystalsIn the most basic of interactions, light refracting through ice crystals produces different effects. Flat plate crystals produce sundogs, while long, column-shaped crystals result in the 22° halo. Credit: Dominique Jacques-Brissette/Scott Sutherland

The 22° halo appears as light passes through the sides of column-shaped ice crystals. The light from each individual crystal is scattered anywhere from 22 to around 50 degrees, and the random orientation of these crystals means that scattered light enters our eyes (or the camera lens) from every direction. Thus, it appears as a circle around the Sun. The circle has a distinct bright inner edge but a diffuse outer edge, and this is due to the hexagonal shape of the crystals.

Most of the sunlight entering one of the six sides of the hexagon is deflected at angles close to 22 degrees, which produces that bright inner edge. Light scattered at higher angles forms the diffuse outer edge. No matter what, though, no rays can exit the crystal having been scattered by less than 22 degrees. The crystal's corners represent a hard limit on that. Thus, the interior of the halo is 'empty'.

Parhelia (sun dogs) are produced by the same diffraction of sunlight that causes the 22° halo to appear, but due to light passing through flat plate crystals. The sun dogs only show up in two distinct areas (in the same 'plane' as the Sun), because the crystals are not randomly oriented. They're floating horizontal, so the refracted light that exits the crystals only comes at us from two distinct directions.

The Parhelic Circle, which appears to link the Sun to the parhelia (and can extend all the way around the entire sky), forms due to sunlight reflecting directly from the exterior of the ice crystals. Light that enters the crystals and is reflected more than once before it exits also contributes to this phenomenon.

Glancing upwards from the halo and parhelia, we see two more phenomena, which appear to be perched at the top of the 22° halo. These are the Upper Tangent Arc and Suncave Parry Arc.

Optics-Upper-Tangent-Arc-Suncave-Parry-ArcA subtle difference in the orientation and rotation of these crystals produces different phenomena. Credit: Dominique Jacques-Brissette/Scott Sutherland

Both of these displays form due to sunlight shining through horizontally-oriented column crystals. The sole difference between only one appearing rather than seeing both is how those crystals are rotating.

Crystals that are free to spin only around their long-axis will produce tangent arcs... and yes, the 'upper' means there is also a 'lower' tangent arc, which can be seen at the bottom of the 22° halo, if the Sun is high enough in the sky. Crystals that are only free to spin about their vertical axis — a rare case known as having 'Parry Orientation' — will result in a Parry Arc. 'Parry', by the way, refers to William Edward Parry, the 17th century Arctic explorer who was apparently the first to see one of these arcs.

In this case, we see a 'suncave' Parry arc because the arc has a concave shape (its ends bow towards the Sun). If this display was viewed when the Sun was closer to the horizon, we would have seen the arc's ends bending upwards (convex), away from the Sun. At that time, it would have been called a 'sunvex' Parry arc.

Although it is very faint, there are hints that there may be a Heliac Arc included in this display. This rare phenomenon is due to a combination of two effects. The first is shallow angle reflections from the outside of Parry-oriented column crystals — but only the lower facets. The second is when rays enter through the top facets, bounce off the inner surface of an adjacent facet, and exit out towards the ground. Given the rarity, the faint lines seen in this display may only be a trick of the camera. However, given that other rare phenomena show up in the display (read on), the heliac arc may indeed be present.

Turning our gaze even farther up, the rainbow-hued Supralateral Arc and Circumzenithal Arc come into view.

Optics-Circumzenithal-arc-Superlateral-arcWhen the hexagonal faces of the crystals come into play, even more arcs can be produced. Credit: Dominique Jacques-Brissette/Scott Sutherland

The same mix of column and plate crystals that results in the 22° halo and parhelia is also responsible for the supralateral arc and circumzenithal arc. Rather than the light entering and exiting only through the sides of the crystal, however, in these cases the hexagonal face of the crystals comes into play. These interactions — where light enters through the face and exits a side, or enters a side and exits through the face — cause more diffraction of the light, so it splits into rainbow colours.

The supralateral arc appears to be the same shape as the 22° halo. Sometimes, it is even confused with another phenomenon known as the 46° halo. However, there is one key difference between the two. If you follow the curve of the supralateral arc towards the horizon, it does not continue past the parhelic circle. A 46° halo would, just as the 22° halo does, but the supralateral arc fades out as it reaches that point.

There is an arc below the parhelic circle, but its curve is flipped opposite to the supralateral arc. This is the Infralateral Arc.

Optics-Infralateral-arcFrom this angle of the video, the infralateral arc becomes more apparent. Credit: Dominique Jacques-Brissette/Scott Sutherland

Also, at the sides of the supralateral arc, there are two especially bright regions of the rainbow, which may be Parry Supralaterals. These form the same way as the supralateral arc. Rays of sunlight enter one of the long facets of a column crystal and exit out one of the ends. However, these only show up if this happens with the rare Parry-oriented crystals. According to Wes Crowley, who runs the Atmospheric Optics website, this apparently makes them exceedingly rare.

On Jacques-Brissette's initial pan towards the Sun, and again near the end of the video, she spots another rare phenomenon, known as the Subhelic Arc.

Optics-SubhelicArc-labelledA rare subhelic arc angles from lower right to upper left of this screenshot. Credit: Dominique Jacques-Brissette/Scott Sutherland

Just outside the supralateral/infralateral arcs, a long white arc starts off low in the field of view and angles upwards into the sky towards the left, intersecting with the parhelic circle on the way. To produce this phenomenon, light must enter a column ice crystal through one end, reflect off the interior surfaces of two of the long facets and then exit out the other end. That's quite the trick-shot, and this arc shows up quite brightly!

Finally, right at the end of the video, she picks up one last part of this display in the last second. Directly opposite the Sun in the sky (partially hidden behind pine trees and with the top cut off) is a Tricker Arc and Diffuse Arcs.

Optics-Tricker-Arc-Diffuse-arcsEven up to the last second of this video, so many amazing atmospheric optic effects are revealed! A rare display of a Tricker arc and Diffuse arcs shows up above and below the anti-sunward end of the parhelic circle. An ankh symbol is added, lower left, to make spotting these phenomena easier. Credit: Dominique Jacques-Brissette/Scott Sutherland

Some of the phenomena captured by Jacques-Brissette have some very interesting (and sometimes rare) requirements. The Tricker arc and diffuse arcs are perhaps the most bizarre, though. Produced by the same type of column ice crystals that result in the 22° halo, a Tricker arc forms when a ray of sunlight enters and leaves the column by the same end face. To perform this stunning feat, though, requires the light to go through multiple internal reflections, bouncing off the inner surfaces numerous times before exiting again! If the light performs this same multiple-reflections feat, but enters and leaves via one of the column's long facets, it produces the diffuse arcs.

Want to see more of these fantastic atmospheric optics phenomena? Winter is most certainly the best time to look for them. So, when temperatures drop and the Sun is shining, look up. You may see something amazing!

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