Saturday, 20 February 2016
Halos from oriented pyramids in the spotlight beam
The night of 20/21 January came with oriented pyramid crystal halos also in the beam. The image above is the last breath of a world class odd radius display that got us completely unawares. This is the first time this winter that the odd radius stuff was visually crystal clear in the beam. Colored and solid upper and lower 9° plate arcs were seen, but if they were still present when the camera was placed in the beam, they are nevertheless washed out by the overexposure. As the lamp was at negative elevation of about 5 degrees, that makes the lower 9° plate arc the 23-6 type. The odd radii stuff was all gone in the next shot which would have had the exposure right. In this precious one one still sees the 35° halo and of the oriented pyramids the lower 20° plate arc.
We drove around and switched on the lamp here and there, stubbornly trying to get a rerun, but to no avail. Diamond dust was already on the retreat, the air was drying up even though it was around -33° C. Yet we managed to get something just before it all evaporated, as shown by the first image in the gallery. A little simulation study (second image) was necessary to see whether it was column or plate pyramid stuff, but that did not give definitive answer. The plate scenario simulations are identical except that in one is 22° halo and in the other 23°. The stuff immediately inside the 22-23° halo and the 35° stuff was noticed initially visually.
That stacking series was taken at 3:30 am. The last two photos in the gallery are the first photos of the night at 6 pm. We photographed these lunar and beam displays simultaneously. No odd radii yet at this early hour.
Marko Riikonen / Jarmo Moilanen
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