ice halos and other optics

Halos 31. May – summary of the month’s halos

Altocumulus virga usually makes pillars. On rare occasions other halos are also seen in it. Yesterday Ac turned into virga for several hours and in the beginning unconventional halos were seen in it: upper tangent arc, circumzenith arc and also short patch of 46° supralateral arc. Parhelion occurred on one shot occasion in contrail. It is getting to look like parhelia in contrail are not that rare after all.

  • parhelion
  • 22° upper tangent arc
  • 46° supralateral arc
  • circumzenith arc

Anyways, this is it. For one month I tried to see every halo possible. I am not going to continue photographing all displays, will just mark up the halos. Next it is time to see what kind of statistics I got for this one month.

Ok, the numbers for May. Halos occurred on 27 days and for individual halos the numbers are:

  • 22° halo – 16 days
  • pillar – 16 days  (including one borderline case)
  • parhelion – 15 days
  • 22° tangent arc – 15 days (upper tangent arc in all and in three also lower)
  • circumzenith arc – 6 days
  • parhelic circle – 2 days
  • 120° parhelion – 2 days
  • 46° lateral arcs – 2 days (supralateral on both, infra on the other)
  • 46° halo – 1 day
  • 9° halo – 1 day

I actually saw cza only on 5 days, but missed pretty certainly one when I was inside. Jari observed that one and I also saw parhelia in that cloud when it had already passed overhead. The numbers above are meant to be absolute numbers of halos, so I added that cza because I most certainly would have seen it had I been outside half and hour earlier or so.

Sofar the halo statistics people have made mostly reflect what they happen to see when they look up. Here I tried to see every halo during the day. I woke up in the morning to see the sunrise and stayed up untill sunset on most days. It became evident that pillars and also parhelia are often seen in very low sun situations in the high cloud bank at the horizon. It may very well be that 22° halo is not the most common halo in Finland, but rather pillar is. 22° halo should have stronger position in the summer when sun spents plenty of time at high elevation when pillar can not be seen, but pillar should gain the upper hand towards the winter when sun is low and when also low level crystals make just pillars. So, as even in the summer month of May the pillar was equal to 22° halo, I would expect in the year’s run the pillar be the most numerous halo.

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