Monday, 23 October 2017

Halo on the night of 15/16 December 2016 in Rovaniemi


Fog turned into diamond dust in an optimal range of -5 to -10° C. I photographed the display in the evening soon after dark, the first photo was taken 16:17. The fog lasted whole night and no doubt the show would have continued whole night too were not the guns shut down soon after 17h, finishing the party before it had even got going. Had there been until morning to play, I am sure wondrous things would have been possible. Actually, there would have been three more nights, as fog continued envelop the city pretty much constantly for three more days and nights. 


In the image above and less clearly in the one below it looks as if instead of "subanthelic 46° supralateral arc" there is more a "subanthelic cza". If so, that would be just a segment of sub-Kern, but it has an unusual location as we are used for sub-Kerns placing most of their weight at lateral locations with an intensity minimum directly above the subanthelic point. Simulations offer a solution, however: if, instead of plate oriented crystals, Parry oriented crystals of triangularish shape are used, a sub-Kern segment is born that is centered over the subanthelic point and looks like a kind of "subanthelic cza". Allow the crystals to rotate a bit and a halo segment much in the liking of what appears to appear in the photo is formed. As a rudimentary demonstration, below are further simulations with increasing rotation of the column shaped triangularish crystals, showing how the sub-Kern turns into a "subanthelic 46° supralateral arc" (ray numbers have been increased with increasing rotation to keep simulations of roughly the same intensity). Parameter file is given at the bottom.  


5 comments:

  1. I see the blue edges very well too and in the bridge display sub-horizon supralateral arc

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  2. Right Michael, there are couple of blue edge effects there. I see three obvious ones, and two less so.

    This, like the one night before, is not a bridge display. The camera was on tripod at 2 meters height on a level ground on top of the bank of the golf field pond. It was two meters or so from where the bank starts sloping down to the pond where the lamp was. With the lamp placed in the pond depression, the level ground is in lamp shadow, which is good for the visibility of halos against the ground. The Wegener/Hastings that is seen in the first image br version is formed against the level ground in a very small air space. Looking at this image I feel every time frustration: the image screams for a full frame sensor. Hopefully others will take this winter similar - and better - photos with full fisheye. In the Czech mountains, for example, where massive displays seem quite commonplace.

    There is also sub-horizon infralateral arc (crossing the sub-horizon supralateral arc on the right side) which seamlessly extends the normal infralateral arc.

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  3. The stuff you have been getting is the stuff I saw in simulations but I never dreamed they would be photographed

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  4. Talking of simulations, here are the normal and subhorizon 46° lateral arcs for a couple of light source elevations:
    http://www.taivaanvahti.fi/js/AjaxFileManagerTinymce3.4/uploaded/SImutaulu%20riikonen%202.jpg
    Red is the normal lateral arc, blue the subhorizon lateral arc.

    A little correction to my post above. That spotlight halos are formed in small airspace is not really anything special. The spotlight displays probably form anyway in small space around the camera. They have mainly 2D character and only rays that come pretty much directly towards the camera contribute to the 2D instances. The shorter the distance between camera and light, the smaller this space is, maybe just 20 cm or so. So the ground a few meters from camera in the line of a halo should not be cutting away any significant contribution. The closeness of the ground rather hampers halo resolution by making the background more noisy with its irregularities and variations of snow reflectance. The Wegener/Hastings and Parry arc & co are seen in the main image against a snowy ground which I had messed up with my Sorel Glacier boots. Had I not disturbed the ground, the halo resolution would have been better. This seems particularly evident in the images taken towards anthelic/subanthelic region. So, no walking around to warm up when doing the spotlight. Just make a line away from the camera (you normally need to get away because of the exhalation clouds).

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  5. That is the halo complex I dream of seeing. If there was a ski resort close by I would be doing your stuff as well.

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