When you do spotlight, two things are desirable: a dark location and stable swarm. On the night of 17/18 December in Rovaniemi both were realized, totaling me more than two hours of exposure.
The sky had cleared up in the daytime and for the longest time I saw the sun, only its upper half at noon visible above the horizon from my 6th floor apartment. Some fingers of fog started to rise also here and there, promising a good halo chase for the night.
When I arrived to the slopes at 4 o'clock these fogs had disappeared and there was no diamond dust. Weather readings put the airport at -12 C while railway station was at -11 C. The airport being at a higher elevation (200 m / 650 ft) than the railway station (80 m / 260 ft), this means no inversion. Only when the temperature roles switch can diamond dust grow in clear sky conditions.
This happened after two hours, at 6 o'clock. The swarm spread south-west and engulfed the city center and the industrial area beyond. It was not worth to start photographing, though, given the light polluted locations and poor material – streelights had only pillars, no parhelia or tangent arcs.
I was hoping for the swarm to reach the southern end of the industrial district, so I could photograph in the large area there used by some earthmoving company. This location in Anttilanvaara is reached by an unnamed road and has a great dark segment spanning about a half a sky.
Finally the swarm engulfed the Anttilanvaara area and I got to work. By this time the temperature had plateaued in my tripod leg thermometer to around -20 C , and the stuff was mostly plate, with some Parry / column in the beginning. Below are shown stacks from the photos I took during the night.
As the display of the previous image deteriorated, I looked around and changed to a place where more elevation was possible. Judging by the location of the subsun relative to the 22 halo, the lamp is about 15 degrees below the horizon. 54 x 30s.
After shopping I headed back to Anttilanvaara to see it was engulfed again. This gave me this best set of the night before the swarm receded again. The edge moved several kilometers towards the city and I decided wrap it up. That was around 2 am. I think I should have just stayed: when looking from my window at 4 am as I was going to sleep, I could see the diamond dust cloud still resided in the same area over the city and possibly it might have extend its reach to Anttilanvaara yet once more. 57 x 30s. The lamp is here maybe about 30 degrees below the horizon.