Tuesday, 29 August 2017

Halos on the night of 14/15 December 2016 in Rovaniemi, part I

Last mid-December, Rovaniemi was shrouded in fog for five days in a row. With temperatures hovering in -5 to -10 C range, the conditions were as good as one can hope for. Just add icy nuclei from snow guns in the air and fog is guaranteed to freeze into a violent display.

The five day diamond dust feast did not materialize. The guns were shut down after a couple of hours after the dark of the second night, and then it was just fog. But I was there on those two first nights to take photos, a selection of which I am going to show in this and coming posts.

I start by two photos from the first night of 14/15 December. Both have, inside the Tricker arc, a faint colored arc, best visible in the blue-minus-red versions. Upon first becoming aware of this feature, I thought, with some excitement, that it might be an exotic halo. This state of mind did not last long, however, because soon the arc turned up in a simulation.

The halo is born from raypaths 3162 and 3152 in column oriented crystals. By its appearance it is a vertically mirrored copy of the more commonly photographed 361/351 arc, which is also seen in the photo (and which has been treated in an article by Walt Tape). At the upper right of the above image is simulation where these arcs are marked, respectively, by left and right pointing arrows. Two column oriented populations were used. The 3152/3162 arc is made by the population that rotates 20 degrees, not by the fully rotating population (see parameter table below). At the lower right of the above image is another simulation, where the the 3152/3162 arc from the limited rotation population is shown without other halos.


  1. I have seen that in simulations and for a long time I was calling an antisolar supralateral arc

  2. I think anyone who has been spending some time playing with simulation software can not have avoided to come by the 3152/3162. I was aware of it too, but yet when I saw it in the photo, I wasn't prepared.

    Anyway, what you have called it could pass for a more proper name of this halo, Michael. Just let it alternatively be called also subanthelic-supralateral arc. So we would have four halos nicely named:

    supralateral arc
    subhelic-supralateral arc
    anthelic-supralateral arc
    subanthelic- / antisolar-supralateral arc

    An objection to such naming scheme might arise from the fact that 351/361 and 3152/3162 are, theoretically, fully enclosed egg shaped arcs encircling the helic/subhelic and anthelic/subanthelic points respectively. Tape says in his article the 351/361 be a kind of complement to the supralateral arcs. Same then would be the case with 3152/3162 as regards to the (as yet unobserved) anthelic-supralateral arc.

    But because the 351/361 and 3152/3162 are the only arcs surrounding also subhelic and and subanthelic points (the supralateral and anthelic-supralateral don't extend there), calling them subhelic-supralateral arc and subanthelic- / antisolar-supralateral arc would sound quite ok to me (at least for now, opinion may change later). The naming would also be justified by how they look in most photos.

  3. Hey I tried.

    Did you see the epic pyramidal plate arc display I got earlier this year