Monday, 15 June 2020

Odd radius sub-plate arcs

Odd radius plate arcs have copies on the other side of the horizon, grouped around the subsun. People have been aware of the possibility of such arcs I call them unofficially here as sub-plate arcs at least from the 1990's, but no observations have surfaced.

Here I show three odd radius sub-plate arc displays that I photographed in Rovaniemi in the winter 2016/2017. The first display was seen on the night of 12/13 December 2016. Only one odd radius sub-plate arc was visible, the 18 sub-plate arc. Below are photos and animations of its appearance.

Flashing two stacked images from a continuous photo series, the other half which had subparhelia and the other which had 18 sub-plate arcs dominating.

Versions of a stack with a slightly different set of photos than above and a simulation. Also other odd radius halos are visible: 18 plate arc and 18 halo, 35 plate arcs and faint 35 halo.

An animation of transformation from subparhelia to 18 sub-plate arc.

A stack of photos from another stage in the display. We see here three halos flanking the subsun: 18 sub-plate arc, subparhelia and a mysterious arc outside the subparhelia. The last one seems to be part of a longer arc that extend faintly to normal parhelion and reminds me of the arc in the 28/29 December 2016 display in Rovaniemi. 

An animation showing the changing display.

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Next I got odd radius plate arcs on the night 4/5 January 2017. This was the winter's coldest night in Rovaniemi. At the location which was the same as above, the Oikarainen gravel pits the temperature was around -35 C. This also was the night with the best sub odd radius stuff: in addition to 18 sub-plate arc there was an upper 23 sub-plate arc. Both were easily visible to the naked eye.

The best odd radius stage did not last long, here are shown three and five images stacks. The display is all but straighforward case. Like in the first display, there are three halos flanking the subsun: 18 sub-plate arc, subparhelia and, well, something. The vertical feature at the horizon should, according to the shown simulation, contain both 35 plate arcs and 35 sub-plate arcs, and while there may be both or either one, that is likely not the full explanation. See again the 28/29 November 2016 display, which is not an odd radius display, but which nevertheless contains a similar kind of thing. Yet another detail to pay attention is 20 sub-plate arc, which is in the simulation, but absent from the display. It was difficult to avoid it in simulation, it should have been in the display. Concerning the lack of normal parhelia and 18 plate arcs in the display, that may be explained by the lamp not being centered exactly on the camera, but shooting slightly over it. I am not sure, this is something to pay attention to in the future (hopefully the nightly diamond dust halo chase is not dead, as it seems now).

This single photo has the 23 sub-plate arc (arrow) somewhat better than in the stacks above. I turned camera here to try catching one of the distant, unobserved odd radius arcs that I had seen in simulations. Alas, no success. After this photo the odd radius stuff deteriorated quickly, so there was no getting a stack that might have actually gleamed out something novel.

Even though the odd radius halos deteriorated, and the display started turning into a normal plate display, here 18 sub-plate arc is still visible inside subparhelia.

Another photo from another place that night with odd radius stuff. We see odd radius parhelia and circular halos at 18, 20, 24, 35 and a possible odd radius helic arc. No odd radius sub-plate arcs here.

And an animation showing the transformation from conventional display to an odd radius display.

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The third and the last odd radius sub-plate arc occurrence was in the display on the night of 9/10 February 2017. Only a weak 18 sub-plate arcs were found from photos, as shown below. Again, the location is the Oikarainen gravel pits. It is a good place, with plenty of possibility to play with lamp elevation. It is quite far from the ski center, though, 12 km as the crow flies, so the diamond dust can reach there only on the coldest nights.

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Taken that odd radius sub-plate arcs were found in three displays in one winter, they can't be regarded much of a rarity just get to the right place, switch on the spotlight, and you got them bagged. With spotlight it is in general much more easier to catch subhorizon halos (well, any halos, really) than in solar displays. In case of the odd radius sub-plate arcs, the subhorizon spotlight method has also the advantage that, because of how pyramid crystals orient to keep the larger basal face as the upper face, these halos are easier to form than in sun light, the latter way of which requires the inner reflection to take place from the smaller, and thus less effective, down facing basal face.

For the last, I add here a collection of photos from first two of these displays, and the 28/29 December display (the bottom row), to more easily compare the features in them:

All simulations in this post are made with HaloPoint2.


  1. I have seen these in simulations but had no idea that had been observed till now

  2. Sorry about this, these lingered a bit too long in the drawer. I still got some nice unpublished stuff from that winter, but no new halos anymore.

  3. Those animations are so cool. Thanks for sharing them Marko.