APOD: Extraordinary Solar Halos (2018 Dec 21)

[APOD Robot]

Extraordinary Solar Halos

Explanation: Welcome to the December Solstice, the first day of winter in planet Earth's northern hemisphere and summer in the south. To celebrate, consider this extraordinary display of beautiful solar ice halos! More common than rainbows, simple ice halos can be easy to spot, especially if you can shade your eyes from direct sunlight. Still it's extremely rare to see anything close to the complex of halos present in this astounding scene. Captured at lunchtime on a cold December 14 near Utendal, Sweden the image includes the relatively ordinary 22 degree halo, sundogs (parhelia) and sun pillars. The extensive array of rarer halos has been identified along with previously unknown features. All the patterns are generated as sunlight (or moonlight) is reflected and refracted in flat six-sided water ice crystals in Earth's atmosphere. In this case, likely local contributors to the atmospheric ice crystals are snow making machines operating at at nearby ski center.

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[Odysseus]

Why does the supralateral arc not appear circular? I assumed that, like rainbows, these halos continue (In a way) beneath the horizon and were always symmetrical.

[neufer]

Why does the supralateral arc not appear circular?

I assumed that, like rainbows, these halos continue (In a way) beneath the horizon and were always symmetrical.

Because supralateral and infralateral arcs involve horizontally oriented, rod-shaped hexagonal ice crystals

Colourful supralateral and infralateral arcs form when rays pass between the side and basal (end) faces of singly oriented hexagonal columns. https://www.atoptics.co.uk/halo/supinf.htm The filtered right hand simulation shows only rays which have passed through an end face and so isolates the supralateral (upper) and infralateral (lower) arcs. The unfiltered simulation at left also has a bright upper tangent arc and parhelic circle produced by the same column

---

<<A supralateral arc is a comparatively rare member of the halo family which in its complete form appears as a large, faintly rainbow-colored band in a wide arc above the sun and appearing to encircle it, at about twice the distance as the familiar 22° halo. In reality, however, the supralateral arc does not form a circle and never reaches below the sun. When present, the supralateral arc touches the (much more common) circumzenithal arc from below. As in all colored halos, the arc has its red side directed towards the sun, its blue part away from it.

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides. Supralateral arcs occur about once a year.

Due to its apparent circular shape and nearly identical location in the sky, the supralateral arc is often mistaken for the 46° halo, which does form a complete circle around the sun at approximately the same distance, but which is much rarer and fainter. Distinguishing between the two phenomena can be difficult, requiring the combination of several subtle indicators for proper identification.

In contrast to the static 46° halo, the shape of a supralateral arc varies with the elevation of the sun. Before the sun reaches 15°, the bases of the arc touch the lateral (oriented sidewise) sides of the 46° halo. As the sun rises from 15° to 27°, the supralateral arc almost overlaps the upper half of the 46° halo, which is why many reported observations of the latter most likely are observations of the former. As the sun goes from 27° to 32°, the apex of the arc touches the circumzenithal arc centered on zenith (as does the 46° halo when the sun is located between 15° and 27°). In addition, the supralateral arc is always located above the parhelic circle (the arc located below it is the infralateral arc), and is never perfectly circular.

Arguably the best way of distinguishing the halo from the arc is to carefully study the difference in colour and brightness. The 46° halo is six times fainter than the 22° halo and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the rainbow with clear blue and green strokes.>>

[sunson]

Why does the supralateral arc not appear circular?

I assumed that, like rainbows, these halos continue (In a way) beneath the horizon and were always symmetrical.

Because supralateral and infralateral arcs involve horizontally oriented, rod-shaped hexagonal ice crystals

Colourful supralateral and infralateral arcs form when rays pass between the side and basal (end) faces of singly oriented hexagonal columns. https://www.atoptics.co.uk/halo/supinf.htm The filtered right hand simulation shows only rays which have passed through an end face and so isolates the supralateral (upper) and infralateral (lower) arcs. The unfiltered simulation at left also has a bright upper tangent arc and parhelic circle produced by the same column

---

<<A supralateral arc is a comparatively rare member of the halo family which in its complete form appears as a large, faintly rainbow-colored band in a wide arc above the sun and appearing to encircle it, at about twice the distance as the familiar 22° halo. In reality, however, the supralateral arc does not form a circle and never reaches below the sun. When present, the supralateral arc touches the (much more common) circumzenithal arc from below. As in all colored halos, the arc has its red side directed towards the sun, its blue part away from it.

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides. Supralateral arcs occur about once a year.

Due to its apparent circular shape and nearly identical location in the sky, the supralateral arc is often mistaken for the 46° halo, which does form a complete circle around the sun at approximately the same distance, but which is much rarer and fainter. Distinguishing between the two phenomena can be difficult, requiring the combination of several subtle indicators for proper identification.

In contrast to the static 46° halo, the shape of a supralateral arc varies with the elevation of the sun. Before the sun reaches 15°, the bases of the arc touch the lateral (oriented sidewise) sides of the 46° halo. As the sun rises from 15° to 27°, the supralateral arc almost overlaps the upper half of the 46° halo, which is why many reported observations of the latter most likely are observations of the former. As the sun goes from 27° to 32°, the apex of the arc touches the circumzenithal arc centered on zenith (as does the 46° halo when the sun is located between 15° and 27°). In addition, the supralateral arc is always located above the parhelic circle (the arc located below it is the infralateral arc), and is never perfectly circular.

Arguably the best way of distinguishing the halo from the arc is to carefully study the difference in colour and brightness. The 46° halo is six times fainter than the 22° halo and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the rainbow with clear blue and green strokes.>>

------------------------------------------
Any diagrams anywhere? THANKS!

[sunson]

Why does the supralateral arc not appear circular?

I assumed that, like rainbows, these halos continue (In a way) beneath the horizon and were always symmetrical.

Because supralateral and infralateral arcs involve horizontally oriented, rod-shaped hexagonal ice crystals

Colourful supralateral and infralateral arcs form when rays pass between the side and basal (end) faces of singly oriented hexagonal columns. https://www.atoptics.co.uk/halo/supinf.htm The filtered right hand simulation shows only rays which have passed through an end face and so isolates the supralateral (upper) and infralateral (lower) arcs. The unfiltered simulation at left also has a bright upper tangent arc and parhelic circle produced by the same column

---

<<A supralateral arc is a comparatively rare member of the halo family which in its complete form appears as a large, faintly rainbow-colored band in a wide arc above the sun and appearing to encircle it, at about twice the distance as the familiar 22° halo. In reality, however, the supralateral arc does not form a circle and never reaches below the sun. When present, the supralateral arc touches the (much more common) circumzenithal arc from below. As in all colored halos, the arc has its red side directed towards the sun, its blue part away from it.

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides. Supralateral arcs occur about once a year.

Due to its apparent circular shape and nearly identical location in the sky, the supralateral arc is often mistaken for the 46° halo, which does form a complete circle around the sun at approximately the same distance, but which is much rarer and fainter. Distinguishing between the two phenomena can be difficult, requiring the combination of several subtle indicators for proper identification.

In contrast to the static 46° halo, the shape of a supralateral arc varies with the elevation of the sun. Before the sun reaches 15°, the bases of the arc touch the lateral (oriented sidewise) sides of the 46° halo. As the sun rises from 15° to 27°, the supralateral arc almost overlaps the upper half of the 46° halo, which is why many reported observations of the latter most likely are observations of the former. As the sun goes from 27° to 32°, the apex of the arc touches the circumzenithal arc centered on zenith (as does the 46° halo when the sun is located between 15° and 27°). In addition, the supralateral arc is always located above the parhelic circle (the arc located below it is the infralateral arc), and is never perfectly circular.

Arguably the best way of distinguishing the halo from the arc is to carefully study the difference in colour and brightness. The 46° halo is six times fainter than the 22° halo and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the rainbow with clear blue and green strokes.>>

------------------------------------------
Any diagrams anywhere? THANKS!
Explaining which is which and what is what.

[MarkBour]

Stunning !

I wonder what kind of lens shape was used for this image. The far left and right portions of the images are angled quite a bit (the building and trees).

Perhaps there is a future niche in the snow-making business that would craft crystals in certain "designer" shapes, then you could stand in the area of the snow blower and see some unnatural refractive effects. Actually, I wonder what possibilities that might include. Surely, someone could create different positions (like a 30-degree halo), but perhaps not new shapes (like a square halo).

[neufer]

Any diagrams anywhere? Explaining which is which and what is what.

Colourful supralateral and infralateral arcs form when rays pass between the side and basal (end) faces of singly oriented hexagonal columns.

---

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through the hexagonal flat top and then exits out the back end.


Infralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals 1) through front end, 2) does a single internal reflection off one of two hexagonal prism sides and then 3) exits out the other prism side.>>

[Ann]

Stunning !

I wonder what kind of lens shape was used for this image. The far left and right portions of the images are angled quite a bit (the building and trees).

Perhaps there is a future niche in the snow-making business that would craft crystals in certain "designer" shapes, then you could stand in the area of the snow blower and see some unnatural refractive effects. Actually, I wonder what possibilities that might include. Surely, someone could create different positions (like a 30-degree halo), but perhaps not new shapes (like a square halo).

These effects require quite cold weather, and it was cold in northern Sweden when this picture was taken.

Don't try to produce these solar halos when it's 0^o C (32^o F) outside.

Ann