Halos 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.  

Close to melting point: a case of high temperature diamond dust

Melt waters draining into sewers and flies warming up on the building walls. You wouldn't expect diamond dust on an such early spring day, but that's just what happened on 6. April in the south-eastern Finnish city of Joensuu.

As I came out from Sokos department store where I went to buy herring (which they didn't have), I was surprised to see ice crystals glittering against the town hall building and ground, making a vague spread out subsun on front of me. For a second I thought of the possibility of tree twigs shedding off their snow (a couple of years ago a guy stacked subsun formed that way) but it did't feel right because for one thing there was no more snow on trees. So I looked up and saw in the sun direction myriads of crystals speeding past in the light wind. This was true diamond dust: the fog that had settled previous day at the darkfall over Joensuu was now clearing up and trying to turn into ice.

Next my eye caught parhelia. They were visible intermittently for 25 minutes until the fog evaporated away. I tried to see cza but could not spot it. Sun elevation during the observation rose from 29 to 30.5 degrees, which is near the end of cza visibility range at 32 degrees. If the crystals were optically below par, as seemed to be the case judging from the rather tame appearance of parhelia, that may explain the absence of cza at such an unoptimal sun elevation.

I don't carry thermometer, but at the official weather station some 1.5 km away temperature changed from -0.2 to -0.3° C (31.6 to 31.5° F) during the time parhelia were seen between 10:55 and 11:15. Joensuu is flat over large areas and I wouldn't expect much temperature differences between my location by the river and the weather station, particularly in fog situation where temperatures should be evened out. If anything, where I was it should have been warmer, because I could see that the station direction was fully enshrouded in fog whereas I was at a transitional area between total fog to the south and completely clear blue sky to the north and thus air must have been warmed up slightly by the sun. By 11:30 it had cleared all around and the temperature at the station was already +1.3 C, thus testifying on the warming effect of sun.

A vertical temperature profile would of course have been interesting as the mass of the crystals seemed to form a bit above the ground. Possibly it was a bit colder up above. On the other hand, the ceiling of the fog has been all the time in full sun shine, so it is hard to say. Maybe a someone versed in meteorology would have an insight on this.

An earlier halo observation near melting point is that by Jarmo Moilanen in Oulu on 3. November 2013. He saw parhelia, subsun and subparhelia at -0.4 to -0.6° C (31.3 to 30.9° F), but by the time Moilanen got camera ready the display had already faded and there was only subsun visible.

View towards south where it was full fog over the lake

Halos and fogbow in the same mix

To photograph a fogbow simultaneously with halos one's best bet may be the foggy autumn mornings with high clouds in the sky. (I leave the possibility of an exhalation fogbow unmentioned).

That should be easy enough. However, connoisseurs might demand the criteria to be that both phenomena must appear in the same cloud. That's making the task a good deal harder. Below freezing temps fogs sometimes have a meager few crystals thrown in the mix and there may be a pillar visible together with fogbow, but anything resembling a proper display is highly uncommon in my experience.

Actually I have never seen one - not until the night of 12/13 December 2016. On that night I was photographing halos at Jokkavaara gravel pits east of Rovaniemi at around -27°C. After the display took a worse turn I went for a little ride to see how things looked a kilometer or so in the city direction. It was the same crap and I returned to the gravel pits.

What a surprise it was to see the place now enshrouded in thick fog. Having just reconnoitered the surroundings I believed diamond dust was right behind the corner, and with the wind being towards the gravel pits, I wondered if the fog might soon start freezing.

True enough, crystals started glittering among the matte fog droplets and it didn't take long to have passable parhelia in the spotlight beam together with fogbow. The more halos intensified the fainter the fogbow became as diamond dust ate away on the water droplets.

It would certainly have been a splendid display if the process had gone all the way. Water cloud born displays tend to be violent. But the fog got an upper hand and situation was reversed to where it started from: only fogbow in the beam and no crystals, no halos. 

So this was a uniform mix of fog droplets and ice crystals. More common are situations where wafts of pure diamond dust and pure fog are alternating in the slight breeze. If you are taking, say, 30 second exposures, an illusion of fogbow being visible at the same time with halos can be created in the image when they in reality occurred at separate times.

Now what if only a display from photons of sun would satisfy the connoisseur? That's really amping up the challenge. There do exist some reports of simultaneous halos and fogbows from polar explorers more than century back, at least from Henryk Arctowki. The only photographs may be those by Ed Stockard in the Greenland Summit Station. But it is not certain if in this case the stuff occurred as one mix or the crystals and fog droplets were as separate layers.