APOD: Unusual Light Pillars Over Latvia (2009 Jan 12)

[jayscheuerle]

These aren't pillars and they don't fan. They're intersecting arcs that "fan" on the bottom too, but because the sky is darker, you see them better up there. - j

[liquidsquid]

Now that several people on this forum have agreed with my earlier explanation, I'm going to disagree with myself

Looking more at the extra photos linked from the APOD page, the "fan" section looks too crisp and orderly to be just the result of dispersed orientations of the crystals higher up -- it looks more like an optical phenomenon. I know this is horribly unscientific, so I ran some HaloSim simulations. Have a look at this one:



The pillar and fan parts look pretty close to the photos. I'd encourage people to play around with HaloSim and see if they can come up with a simulation that's even closer to the photos. If this simulation is what's happening, the pillar is caused by horizontally-oriented plate crystals, as I described in my original post, and the fan part is an Upper Tangent Arc, caused by horizontally-oriented column crystals. The shape of the arc is dependent on the altitude of the light source relative to the observer - a higher light source (or lower observer) produces a wider arc. In the simulation, I used a light source 0.2 degrees in size at 8 degrees below the horizon. It looks from the photos like the shots were taken from an upper floor of a building, so having the light source below the "horizon" makes sense from the point of view of the orientation of the crystals.

My simulation file is available here.

This is consistent with what may be going on in the atmosphere. Typically when we have light pillars in our area (WNY), it is super-saturated cold, still air near the ground. The ice crystals (tiny snow) is forming in the super-saturated air near the ground and falling very slowly down flat-face first while growing at the same time. Above this "blob" of cold air near the ground is typically warmer, drier air which has been forced up by the pooling cold air, or is simply over-riding the cold air below due to better mixing, so you will have a relatively sharp transition from cold saturated air to this moderated air mass above.

If you are at a high vantage point in the warmer air, you don't see them at all. As you descend, they suddenly appear as you pass this transition (I live on a hill, and have seen this), and get brighter the deeper into the moist air you go. Though the length appears to extend, it does not mean the pillar is longer, only brighter. As long as the ice crystals are over your head, they will appear to go all the way up. It is an illusion that they are over the lights themselves. The only reason they fade away at the tops is they are simply not reflecting as much light back to you since they are more and more directly over your head until finally they are behind you.

However I have never seen this style before, but here is what I will guess:
There is a mix of flat ice crystals as normal being formed in the still air near the ground which create the normal light pillar which is common, and also other crystals formed higher-up in a different set of conditions, perhaps where there is a good mixture of turbulent saturated air so the ice forms a different shape. The normally sharp transition may not exist in this case, and there happens to be a fairly strong wind mixing air up a few hundred feet. The two shapes are blended near the observer in the still air, and depending on their perspective into the mix of shapes, depends on which shape has best reflectivity. A greater angle between the observer, ice, and light is more reflectivity from the ice formed higher up, the less the angle, the more from the flat lower-formed crystals.

The crystals are not changing shape much as they fall, other than growing.

-Mark

[salmo]

The density would be of higher concentration near the ground and peak at some height with crystal orientation then with less density at the top and a more flat crstal orientation you would get fanning at least that's my best hypothesis

[aristarchusinexile]

The fanning out is caused by an upper layer of colder air cooling the relatively warm columns rising from the lights, causing them to fan out as a beginning of convection, if convection is the right word, the fanning out being the first stage in the cooling air's return to lower altitude, returning to the ground so to speak. The columns, or pillars, have been featured on Apod before, and I seem to recall the processes are well understood, involving ice crystals forming from water vapour of course, and prismatic reflection, with the density of the ice crystals allowing light to reflect back and forth between crystals. Of course, on the atomic scale, is the light 'bouncing' off crystals, or do the photons excite the atoms in the crystal to give off their own light. I understand that is how radar operates, an energy wave from a transmitter exciting the molecules in the skin of the aircraft to emit its own radio signature, those waves transmitted to a receiver. Dear Physicists, is it the same mechanism here? Or simply reflection, if such a thing as simple reflection exists?

Of course, in this case, an equilibrium of temperature at the point of fanning may cause the crystals to simply hang in the air, but one would expect a diffusion, a more horizontal structure. I think it's fairly safe to say that if the photo was a movie we would see the fanning crystals falling, continuing to the ground .. and in fact it would be interesting if at a later stage in this process the light would be seen as a circle falling from the fan's outer edge to the ground, and beginning possibly to rise as the air is warmed at ground level. Warming air would not necessarily cause the crystals to melt .. depending on temperatures.

[logmark]

Since this is photographed at night I suspect an exposure of some duration. If, during the exposure time, slight air currents tilted the horizontally aligned ice crystals a bit left then right (or if they were falling similarly to autumn leaves) the reflections would then appear to come from directions to the left or right of the light source. Keeping in mind that the column doesn't actually shoot up from the light source, but exists all along the vertical plane between the viewer and the light source with the highest part of the column reflecting from closer crystals, there is plenty of horizontal distance for air currents to tilt these bits of ice one way and/or the other at differing distances from the camera. I've watched similar columns waver left and right slightly and think that if a long exposure had been made of them, similar images may have
resulted.

[MCinFL]

Simplest solution: Ice pillar crystals are real.

The photograph shows all objects, not just light, but poles, trees and columns, converging and then expanding above a certain portion of the photograph. Solution is that the photograph has been put through a fisheye to rectilinear converter (digital) and the distortion at the top is a photographic artifact, not one that would be visible to the naked eye.

[Dominic Bercier]

I would like to propose a reasoning to the light pillars photo. The answer to me is twofold. If we were in the dead of space, light from any source would venture in all directions but in Earth's atmosphere it must contend with 1) water particles and 2) gravity, which both affect the light. What if the ice crystals are vertically oriented, and create tiny microscopic mirrors funnelling the light in an upwards restricted (and downwards also) direction, which give us the pillars. Any light projected from approximately 1 to 179 degrees would naturally bounce off the ice crystals in this light chamber and be caught in the mirror, creating the pillars. However it is the light shooting straight up that could give us the arches at the top of the two main pillars pictured here in the Latvia photo since it suffers Earth's gravity (unlike particles in dead space which would be encouraged by similarly oriented crystals into a perpetual pillar instead). Though reaching upwards as projected light particles, the gravity from our planet is pulling the light particles back down, creating a loss of speed and focus in the particles reaching straight out for space at no angle, and though the particles may never touch back down on the planet like most heavy matter, its tendency back towards the Earth creates an arc as it warps out and into space. This is likely the natural phenomena we see from all sources of light on our planet but cannot tell from a source in normal conditions, except that it has been highlighted here as the light was raised by the ice crystals into a pillared pedestal that allow us to witness the gravity affecting light first hand. - Dominic Bercier, writer and illustrator, Ottawa, Canada.

[xstevesmithx]

IMO, it appears to be an incomplete "kern arc" or circumzenithal arc:



They are also seen in artificial lights, such as the image under discussion. This page has many examples of arcs and pillars:

http://www.ursa.fi/blogit/ice_crystal_h ... p?m=200801

Steve Smith

[aefields]

pmason has the answer. As I observed earlier, the shape is like the upper tangent arc, but sharper than any examples in the photos I've seen. I didn't realize the angle above or below the observer's horizon made the arc sharper or more obtuse. That explains it perfectly. Comparing photos with lights above and below the observer should adequately prove or disprove the theory.

As some have mentioned, difference in air layers doesn't explain it since all the arcs are at the same angle - not the same height.

[liquidsquid]

pmason has the answer. As I observed earlier, the shape is like the upper tangent arc, but sharper than any examples in the photos I've seen. I didn't realize the angle above or below the observer's horizon made the arc sharper or more obtuse. That explains it perfectly. Comparing photos with lights above and below the observer should adequately prove or disprove the theory.

As some have mentioned, difference in air layers doesn't explain it since all the arcs are at the same angle - not the same height.

You are right, the only difference the air layers make is the shape of the crystals being produced. With more mixing from turbulent air (upper layer), crystal formation will be different than that of still air (near ground). Thus the two types of reflections mixed together.

And NO it is not heat columns over the lights. Most of the light causing the pillars that the camera sees is only a several feet away from the observer, though it is the accumulated light between the observer and the light source which creates the final image. If there were any effect from rising heat over the lights, it would be very minor, and look more like a heat column over a candle (turbulent).

-Mark

[neufer]

IMO, it appears to be an incomplete "kern arc" or circumzenithal arc.

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<<The first photographs of the Kern arc were taken by Marko Mikkilä close to Vuokatti Ski Resort Sotkamo, Finland on 17 November 2007.
The photographs were in natural sunlight taken from an artificial cloud created by SNOWGUNS.

At the Vuokatti Ski Resort about 100 SNOWGUNS may operate at one time.>>
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http://en.wikipedia.org/wiki/File:Minam ... ritomo.jpg
http://au.youtube.com/watch?v=-8RUaFBEuvQ

[numbers_game]

Perhaps the ice bearing layer of air rises adiabatically to its top, which is dimpled, as in mamata. Crystals might reflect
unevenly along occasional irregularities near the top of the saturated air, giving the appearance of a cone.

[DEEarl]

This has been an enjoyable discussion. We are going sub-zero in Minnesota for the next several days. Since these are the conditions that often result in light pillars I will be looking for them, with sharpened observational skills, over the next several nights.

[BMAONE23]

Sorry to hear about the Sub Zero temps there..Santa Rosa Ca was 81 yesterday

[apodman]

http://en.wikipedia.org/wiki/Frostbite_Falls

http://en.wikipedia.org/wiki/International_Falls

[Don Stoner]

There were many good observations posted (and also many entertaining ones): First, logmark's observation that the pillars exist on a vertical plane between the source and the observer. Next, chadair's that the stars provide angular scale. And, of course , PiTHON's detailed observations. Finally, Stephen McDonald's that the "top" was closer than the base. Just how much closer might be a surprise! First the math and physics: light passing through horizontal rod shaped-hexagonal ice crystals refracts vertically at about 22 degrees (the Lowitz angle - from the geometry + Snell's law + 1.31 index of refraction for ice). When the distant sun is involved, this means the effect is observed 22 degrees from the source; but when the source is inside the crystals (like here) there is a whole family of paths which involve a single 22 degree downward refraction. This family describes an arch on logmark's vertical plane with one leg at the source and the other at the observer. The top (peak) of the arch is seen (by the observer) 11 degrees above the source (starts up at 11 degrees, then gets bent down 22 to -11). The arch is a circular arc which is comprised of 44 degrees (360-(180-22)*2, from the geometry) worth of a full circle. The tail-end of this arch hits the observer at the full 22 degree angle (so nothing an be seen above 22 degrees). Since chadair has placed the bottom of the fan at 17 degrees (well past the 11 degree middle) the "top" of the fan appears to be coming "up" on the full 22 degrees- putting it right next to the camera lens. (This effect would not be observable with stereoscopic photography because the "mirage" moves with the observer.) The fanning must be a ground-level effect! Some "bloom" can be seen around the sources suggesting dust or possibly moisture; but since light from all of these different optical paths must pass through this same ground layer, we need another explanation. I'll suggest that the crystals might be "fogging up" (would require exactly 0 deg C.) or that they might be losing their horizontal orientation near the ground. The low temperature probably falsifies the former. What does the "winner" get? I hope it's a cool APOD calendar! -Don Stoner (B.S. physics)

[Don Stoner]

Sorry, I need to learn to write: The source is inside the "cloud" of crystals - Not inside the crystals themselves. -Don

[Don Stoner]

One more thought: The crystals near the ground might be dirtier (4 mph wind with dust) - degrading their optical quality. The other paths (with the exception of the source "bloom") miss these low crystals. -Don

[Don Stoner]

When I plotted it (.gif should be attached) the fan at the top started looking more like simple airborne-dust scattering. The lower the path (both "top" and "bottom" of the pillar), the more near-surface dust it has to pass through and the more likely it will be scattered. -Don

[Don Stoner]

... but now it's starting to look like the "focused" section ought to be eleven degrees away from the source - where the highest and cleanest path is. (There goes my calendar.) Can I get a re-measure chadair? -Don

[logmark]

I'd like to point out that we are talking about evidence of light pollution. Lights that point downward or are otherwise shielded to direct their lighting only toward the ground do not produce light pillars. Only those light sources which permit light in strong sideways or upward directions give us these images. As astronomers (or buffs) ought we not be looking at ice pillars as evidence of offending light sources?

[neufer]

I'd like to point out that we are talking about evidence of light pollution. Lights that point downward or are otherwise shielded to direct their lighting only toward the ground do not produce light pillars. Only those light sources which permit light in strong sideways or upward directions give us these images. As astronomers (or buffs) ought we not be looking at ice pillars as evidence of offending light sources?

The Baltic States are far from being the worst offenders:

[mikedel]

Sometimes one tries to hard to see something or think to hard. I try to keep things simple. To me I believe that the fanning at rhe top is just weather related. It could be that as the ice crystals are falling they are in all different shapes and size. Size being the key word. The smaller crystals meltt before they can hit the ground, while the larger ones continue further on down. So the columns don't fan out at the top but rather the column narrows as the crystals fall. Fantastic pix though.

[kb9hpq]

I would suggest that as the base of the cloud becomes more dense the light is diffused over a broader area.

[mihondo]

This morning here in Midland, Michigan, we saw this effect!
Temperature = 0 deg F, Wind speed: 0 .