Circumhorizon arc in spotlight beam

By Jarmo Moilanen, Nicolas Lefaudeux, Marko Mikkilä and Marko Riikonen

It is not that unusual to see a circumhorizon arc under a streetlight, forming a striking spatial shape. On the night of 6/7 January  we gave circumhorizon arc a shot with spotlight as we arrived to a place where there were rather high piles of logs on top of which we could clamber while leaving the lamp on the ground below.

Whereas one usually just walks into the beam, here it was more a matter of sticking one’s head into the very narrow beam – narrow because the lamp was so close.

The circumhorizon arc was only crystal glitter, but it was conspicuous. The image above is a maximum stack of several individual photos. At the top is the subsun and a little underneath one may discern a vague blue horizontal line – the blue circle.

The blue circle is better picked up by br treatment as shown by the image on the left. The simulation next to it contains also sub-circumhorizon arc. Had we not placed the lamp so low (the simulation is for 67 degrees), we would have probably caught also the sub-circumhorizon arc. The crystals were probably too thin to produce it at this elevation.

The final photo shows the setting. The night was cold and at this location the night’s lowest, -35° C was felt.

Blue subsun

By Jarmo Moilanen, Marko Mikkilä, Marko Riikonen and Nicolas Lefaudeux

On the night of 6/7 January we gave a try also on another halo effect that simulations predict: the blue subsun. It is an effect in the blue spot and blue circle family and it colours the core of the subsun blue at 58.5 degree light source elevation.

We though we failed to photograph the effect and then forgot about it and were surprised when it turned out in other photos where we were actually aiming at subparhelic circle.

The lamp-camera configuration did not correspond with that optimal elevation of 58.5 degrees, but the blue subsun effect is nevertheless seen in outer areas of subsun in the two images we got. In the image above it is visible on the upper edge of the subsun and a matching simulation was found at 60.5 degree light source elevation. The grayscale br image highlights the blue of the subsun particularly well (bright white).

In another photo (below) the lamp was, according to simulation, at 57 degree elevation and the blue colour is on the other side of the subsun. Shown are also crystals collected during the display and uncropped images.

Catching a divergent light halo effect predicted by simulations

Sometimes it is possible to make a deliberate attempt to photograph something predicted by simulations. On the night of 6/7 January we made such an attempt on the diffuse spots of light that in simulations are seen next to the divergent light subparhelion.

The effect is formed by a mixture of subparhelic circle raypaths, including 3157 raypath and sub-120° parhelion raypath. Its exact shape and position depends significantly on the crystal shape, like for the Liljequist parhelia.

To obtain an omnidirectional secondary light source that was bright enough we pointed the lamp directly to the snow surface. We took photos, looked at them more closely the next day, and there it was – those smudges of light predicted by simulations.

The photo above is actually from a slightly better case on the night of 18/19 January. Next to it is a simulation. Below is the one on the 6/7th.

Nicolas Lefaudeux / Marko Riikonen / Jarmo Moilanen / Marko Mikkilä

Odd radius display in spotlight beam

Here is a photo of a diamond dust odd radius display in the spotlight beam. Of the less commonly seen halos visible are lower 20° and upper 35° plate arcs.

The image which is a stack of several photos, was taken in Rovaniemi on the night of 6/7 January. The odd radius stuff seemed confined to this particular location, Rikanaapa bog, to which we paid several visits during the night. From our observations and the photos taken it looks like the display remained there pretty much unchanged through the whole long night. The crystal swarm originated from snow guns 6 km away.

The version above, which shows the halos best, is done with the “blue-minus-red” method. Below is also a version with minimal intervention and a one with an unsharp mask.

The display has lower 24° plate arcs and it seems like there may be the lower 9° too. As the lamp is about 5 degrees below the camera, that makes these plate arcs of the 23-5 and 23-6 type respectively. In other words, the B-components for these arcs, if you will.

Nicolas Lefaudeux / Marko Mikkilä / Jarmo Moilanen / Marko Riikonen

Torch shaped Moilanen arc

The two images above, which are versions of one stacked image, show a torch shaped Moilanen arc. This is the shape that the halo takes when light source is about 9 degrees below horizon. Below is a simulation for that elevation.  Also shown is another photo in which the places of lamp and camera were switched, giving a more familiar looking Moilanen arc with the lamp about five degrees above the horizon. The display was photographed on the night of 5/6 January in Rovaniemi.

Marko Riikonen / Nicolas Lefaudeux

Wegener or Hastings?

This spotlight display that was photographed in Rovaniemi on the night of 5/6 January rises the question of whether it is Wegener or Hastings arc or both together that is seen here. From the outset, as there is helic arc but no subhelic arc one could argue that it is rather the uppervex Hastings arc.

When comparing with simulations (below) it does not look that obvious anymore. The curvature and extent of the arc looks more akin to Wegener than Hastings (the side view image). And it is possible to simulate Wegener without subhelic arc by using thin plates in column orientation. Indeed, to have best agreement with the images, in all simulations thin plates were used both in Parry and column orientation.

In the image on the left the crystal h/d is 0.1 for all simulations. In the one on the right it is h/d 0.03 for rotations of 0.5 and 5 degrees, for full rotation simulation h/d 0.1 was used. Crystals are triangular, oriented in Parry orientation so that the tip is pointing up.

Well, none of these simulations come out satisfying. The light source elevation for simulations is -5 degrees.

Marko Riikonen / Nicolas Lefaudeux

– added one more image with simulation containing both Wegener and Hastings

Diamond dust halos on the night of 12/13 January, part III

Has anyone seen subparhelic circle inside subparhelia? Neither have I. Not before stacking the photos from the latter part of the 12/13 January night. In one of the stacks this missing segment is seen faintly inside subparhelia. The version of the image above, made by Nicolas Lefaudeux, was done with emphasis on getting the segment stand out as clearly as possible.

Below is a more conventional looking version of the image and simulation. The plate oriented crystals in the simulation were fully triangular. Regular hexagons do not make the missing segment, except at very high light source elevations.

What else? The arcs that in Finland these days are called the Schulthess arcs are quite well defined, especially in the last photo set of the night (below). One sees both concave and convex components extending from subparhelia to parhelia, though the latter component seems to have a gap around the horizon, not reproduced by the simulation. I have never seen in sun display photos the concave component between parhelia and horizon. It is always just the convex component that is visible.

Marko Riikonen