Wednesday, 9 December 2020

Divergent light tangent arc on sparsely falling diamond dust

In Cluj-Napoca (Hungarian: Kolozsvár), Romania it is a common winter weather situation to have a dense layer of fog sitting on the city, with temperatures a few degrees below 0⁰C throughout the day with only the surrounding higher hills and mountains rising above this fog. High levels of humidity combined with low temperatures and aerosols floating in the air often results in industrial snowfall or sometimes (especially near the edge of the fog) diamond dust. On the evening of 2nd December we had the above described conditions but the weather was about to change, with gentle southern wind slowly sweeping out the fog.

Around midnight as I prepared to go to bed, I looked through my window and instantly noticed the unmistakable tiny sparkles of diamond dust under the streetlights. It was too sparse to form consistent halos (although the most intense moments produced visible lower pillars), but the sparkles hinted to the presence of the lower tangent arc. After a quick set up, I began taking videos of the area below the closest lamp (at 8-10 m from me, and approx. 8 deg. below eye level). The videos were around 4 min each, but many of them were rendered useless by the sudden drops in the amount of the diamond dust. Between two videos I also snapped a quick picture of a distant lamp that shows the pillar roughly as it was visible to the naked eye (see below).

A clearly visible pillar formed on diamond dust in the light of a distant lamp

On my computer I extracted every frame from the videos and saved them as jpg files (separately for each video), and stacked them together using a StarStax (a free startrail-making software) in ’lighten’ mode, to get the brightest value for each pixel in the resulting image (this is important as averaging the frames would wash out the individual sparkles that make up the halo). With this method I got an integrated image from each video that contained all the sparkles that appeared throughout that video. The videos were taken at various focal lengths ranging from 18 mm to 55 mm. When the first stacking results came out, I was simply delighted to see the lower tangent arc, and did not think much about it. However, on the next day I noticed something fishy: the tangent arc and the lamp comfortably fitted into the same field of view at 55 mm, which was not usual: this arc had to be way closer to the light source than a normal tangent arc. With the help of the solar disk (picture taken with the same equipment and settings, and resized to match the dimensions of the video), I measured and got approx. 7 degrees for the distance of the tangent arc from the lamp (see below).

Measuring the distance between the lamp and tangent arc. The solar disk on the top was used as a reference for the 0.5 deg. unit of the scale

This was completely new to me as I’ve never encountered any observations that describe tangent arcs so close to the light source, but I suspected the divergent light as the cause, since I knew that it can considerably alter optical phenomena compared to their familiar counterparts caused by paraller light rays. With the help of Walter Tape’s detailed article on streetlight halos (1), I confirmed that it is indeed the divergent light that caused the unusual appearance of the tangent arc, its distance from the lamp being normal for the lamp’s elevation relative to the horizon (since my observation of the below-horizon lamp with a lower tangent arc is equivalent to an above-horizon light source with an UTA at same angular distance from the horizon).

Based on Tape’s article, I also concluded that the strip connecting the lamp with the tangent arc is not a simple pillar, but part of the tangent arc. The slight widening at its center corresponds with the figures in the article.

On the following link you can watch a short piece taken from the video used for the picture on which the measurement was made, and the (sped-up) image stacking process using the frames from the original video: (2).

Beáta Ujvárosi


  1. Tape has been proven right once again. Well spotted that small distance from the lamp. There is by the way this older observation of a tangent arc "totem pole".

    That village you live in should have some kind of ice nuclei producing gun. You would see awesome displays during those foggy midwinter periods.

    This winter I have been watching many times tangent arcs from streelights while driving. From the distance, when you are not yet in the swarm, you see pillars that extend only downwards from the lamp. Then, as you enter the swarm the pillar of the streetlight ahead of you starts curving towards you, and last you see before the lamp gets high up over you is the classical instance of the tangent arc. This is then repeated under the next streelight and so on. It is quite dramatic. Now that every lamp pole has these superbright led lights, streelight halos are even more impressive. During the last chase I took some time to just watch good 3D plate under one streelight, I could walk back and forth and let the display elevation change, the Tape diagrams coming to flesh before my eyes. Someone should come here and take videos with good camera.

    1. The observation you linked is amazing! I like how well it demonstrates the different forms of the same halo depending on the elevation of the light source.
      The nearest ski slope is cca. 10 km away in southeastern direction from me, but at the time of my recent observation the snow guns did not operate (there is an online live cam of the slope that I regularly check). However, the city's air quality is usually bad and pollutants often accumulate in high concentration during winter (especially in temp. inversion situations) that can act as condensation seeds.
      I have witnessed some diamond dust events here in the past, they always occur on the foggy/sunny border and the amount of diamond dust is usually not too much, producing only faint halos (pure column displays with tangent arcs at temps around -6C, and pure plate displays with CZA and sundogs around -15C, but I have never seen a mixed one so far). It would be an unforgettable experience to see a stronger diamond dust event with vivid divergent light halos like the one you described above! Unfortunately, I have no chance for anything like this in the coming weeks due to crappy weather (temps above zero and rain, nothing like a normal winter). As January is usually the best month of the season I hope it will bring decent winter weather suitable for diamond dust halos.

    2. It is typical that when fog is turned into ice by pollutants, the action is somewhat restrained. Recently Rovaniemi had two successive nights of fog with temps at around -5 C and guns not on. On both nights I saw little transient swarms with tangent arcs (and also Moilanen on the other night), but it was tame compared to the raw power of snow gun made diamond dusts.

      Here -15 C temperatures are the worst, maybe only pillars, or no halos at all. I have found already earlier, some Czech observation I think, that in the mountain regions of central Europe this temperature regime may actually be very good.

      Having not mixed diamond dusts is a good thing concerning crystal samples. A sample from a display with only column halos and Moilanen arc would be most interesting.

      Last night I was out, temps around -7 C, overcast with stratus. Should have been perfect, but no. Instead of diamond dust guns nucleated the cloud to give off snow fall in the direction of the airport. Pillars were seen occasionally in crystals some distance above the ground. This had likely something to do with the rather high base of the stratus and somewhat windy conditions. It seemed no change was on the cards, so I left the scene at 3 am.