2008. február 8., péntek

Stacking an odd radius display on snow surface

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.

6 megjegyzés:

  1. Stacking 118 images must have been quite a job, but it has brought its reward. The photos are amazing, the halos are really sharp. And now we have the first crystal samples from an odd-radius surface halo display! Congratulations, Marko!

  2. It would not have been much of a job if I had given it to Registax. But Registax can not handle 16 bit images and I did not want 8 bit artefacts to these, so I opted for manual stacking in CS2. Also, I was then able to align the horizon in the photos that had some tilting due to small irregularities in the river ice below.

    Piikki took already crystal photos from ice surface display this winter. But these are the first pyramid crystals from a surface display, if nothing else.

  3. Excellent piece of work! As we know that Jarmo Moilanen has succeeded in photographing surface pyramid crystals and that people have reported pyramid crystal halos in the past (Piikki, Löwenherz), one should have eyes open every time a surface halo appears. The meteorological conditions prior to and during the appearence of pyramid crystal halos should be documented accurately. Maybe these observations could shed some extra light on crystal formation conditions.

  4. Its incredible how sharp and distinct the 24° halo appears in the stacked images. It looks really nice. The pyramid crystals are also beautiful, they remind me of Tape´s books crystal picture quality level.

  5. Michael Ellestad2008. február 9. 10:39

    Wow Marko very nice I like the contract with the halos in the sky and on the ground and the sharpness of the odd radius halos is very nice.

  6. You had a great idea. Very nice and innovative!