Moon diamond dust odd radius halos in Finland

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On the evening of 17 February Tapio Koski went out on the river ice in Muurola, Rovaniemi, to photograph northern lights. They were not good and he took photos of the moon halos that appeared in the sky. It was an odd radius display and it contained two extremely rare halos: the 20° and 35° lower plate arcs, of which the lower 35° plate arc is even more difficult to catch, as it is never seen much above the horizon. Everything went smoothly for the documentation of the display. The location was optimal with low horizon and Tapio was using wideangle which allowed to capture the whole display and he took series of photos on a tripod for stacking.

First two of Tapio's photos above are single frames with only slight unsharp masking. The next three images are four and five frame stacks and have stronger usm to enhance the lower 35° plate arc at the horizon.  In the fifth image the different radii plate arcs are identified. Moon elevation for the photos is 31-32 degrees. More Tapio's photos are on his site.

Odd radius diamond dust displays are rare in Finland. All together about half a dozen cases have been reported since 1983. The only other lunar odd radius display in Finland was captured by the Finnish meteorological institute automatic aurora borealis camera in Kevo, Northern Finland, on 8 January 1985. Above is a drawing of the display by Jarmo Moilanen.

The odd radius diamond dust displays are formed in urban areas from the power plant and factory plumes at low temperatures. While rare in Finland, in Alaska's Fairbanks they are seen regularly every winter due to the cold weather.