Identifying light sources responsible for “floating” pillars

Recently, artificial light pillars were reported from Kuopio, Finland, which machted a city map projected into the sky. Artificial light halos are much less common in the German lowlands, especially in mild stages of the winter.

The more exciting it is, when nonetheless pillars from streetlights appear under such conditions. Of course they do not reach to the bottom then for the lack of crystals near the ground. This happened to me when cycling from a neighbouring village (Meuro) to my home (Hörlitz) in the Lower Lusatia region, on December 06th, 2018, a little after midnight (at air temperatures around -2 °C…-1 °C). My way led through a recultivated open coal mine (at roughly 51.53° N, 13.94° E), and I stopped twice to take photos of pillar segments that were floating in the sky at elevations of about 10°…25° in eastern directions. Their brightness was not great, as they were visible only in the dark outside villages and it took exposure times of about 30 s (at f/3.5 – f/4 and ISO 1600 on a Pentax K-5 camera) for decent photographs. The intensity also fluctuated over typically 30 s, depending on the changing number of properly oriented crystals at the right spot.

(00:33 CET, position A, f = 20 mm)

(00:49 CET, position B, f = 40 mm)

Of course, a very interesting question is where the responsible light sources were located. Under the assumption that the reflecting faces of the ice crystals are perfectly horizontally oriented, the light pillar is confined to a single azimuth coinciding with the the one of the source at the horizon, though the source itself may be rather far off and not visible. Luckily, some stars could be identified in the pictures from both observing positions (A and B), so the azimuths of the pillars can be determined. The brightest white pillar (1) higher in the sky and the brigthest yellow pillar (2) from the lower fence-like structure were chosen for further analysis:

(00:33, position A)

(00:44, position B)

When plotting the azimuthal directions from the two observation locations, intersections at 18.3 km (pillar 1) and 26.5 km (pillar 2) distance are obtained. These fit with the southern edge of the still active Welzow coal mine, and the Schwarze Pumpe power plant.

As the crystal positions, projected to the ground, are located halfway between the observer and the light source (see sketch below), they can be drawn on the map as well. The gray area above the lakes “Sedlitzer See” and “Partwitzer See” marks these locations as deduced from the pillar observations, though of course no actual boundaries of this area can be determined from this simple two-point study.

With the bottom distance d to the light sources known, it is straightforward to calculate the height H of the crystals above ground from the elevation angle h by simple trigonometry. The white pillar 1 in the image taken at 00:44 from spot B extended about 17.6° – 24.1° in elevation, and the yellow pillar 2 about 13.2° – 17.0°. It follows that the crystals were located at heights between 3.0 km and 4.1 km.

It should be added that earlier that night (Dec 5th, 20-21 CET), similar pillars were observed by Sören Petersen at Hohwacht, Schleswig-Holstein, at the Baltic sea. The distance to my location amounts to almost 400 km. Maybe the weather conditions favored the existence of a bigger ice crystal field at that time, as the rarity of such pillar observations in Germany renders a purely random coincidence unlikely.

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