For some years I have been waiting to get to employ stacking technique for snow surface halos. Last weekend the chance finally came along, as a moderate display appeared on two consecutive days. Here I present the results from the first day.
The image above is stacked from 118 images which were taken on a frozen river in Rovaniemi. Between each shot the camera, which was on tripod, was moved some 20-30 cm sideways. This serves to decrease noise of the snow surface in a similar manner as cloud movement decreases the noise in stacking of sky halos.
Already on the spot I thought of seeing double structure in 22° halo, indicative of the 24° halo, but was not certain of that until two hours later, when I came back with microscope and saw an abundance of pyramid crystals among normal crystals in samples taken from snow surface ( 1 – 2 – 3 – 4 – 5 – 6 – 7 ). In the above displayd stacked image the 24° halo is conspicious and also a close up image is given ( 8 ). 24° halo is seen also weakly in single unmanipulated images ( 9 ).
9° halo, which should be always occurring with 24° halo, is not visible in single images, but appeared in stacked image, and is seen perhaps best in an image that has only light unsharp masking ( 10 ). Heavily unsharp masked version resulted in 35° halo appearing faintly ( 11 ). At 86 frame stage I could not yet detect it, so additional 32 images made a difference.
In sky displays 35° halo is always associated with 20° halo because they are closely related in formation. As a thumb rule, one can have 35° halo if 20° halo is intense enough.
Here, however, it seems that there is no 20° halo or at most a ghost of it is present. Possibly the alternate pyramid faces, which make the 35° halo, are represented in the special snow surface situation in significantly greater proportion in relation to opposite pyramid faces, which make the 20° halo.
The lowest temperature measured at the official station in the city center was -12°C at 9 in the morning, three hours before I noticed the phenomenon. This is not quite enough for pyramid crystal formation as far as airborne crystals are conserned, which start occuring below -20°C.
However, down on the river ice away from the city center and especially on the few centimeters above the snow surface (clear sky) it can have been cold enough for pyramid crystal formation. Could be also that snow surface presents a special situation where not as low temperatures are required as with airborne crystals.
The crystal photos are all in same scale, the width of each square is approximately 0.6 mm.
The next day the display looked slightly different but pyramid crystals and odd radius halos were still present. I took 350 photos but have not worked on them yet.