Saturday, 4 February 2017

Do clouds slice reflection subsuns?


A good proportion of reflection subsuns are sliced and there has been discussions of cloud shadowing as one explanation. Recently I observed a sun pillar that was sliced by cloud shadows (image above). This seemed in my mind to concrete the possibility of clouds as a culprit.

Yet at least one observation don't fit that picture. In April 28, 2015, I saw in Kirkkonummi a reflection subsun which slices remained stable too long to be caused by clouds. Three successive photos of this display are shown below. Clouds move and cloud rays change, but the two faint reflection subsun slivers stay unchanged.

Maybe in some other cases clouds can be the explanation, but in the Kirkkonummi display it clearly can't account for the phenomenon. So I guess we are still, if not in shadows, at least in penumbra with this issue. The sometimes observed displacement of reflections subsuns from the solar vertical is another funny feature.

The original report of the Kirkkonummi display.

7 comments:

  1. Marko, have you calculated the source of light? Was it the sea? Understanding from your earlier blogpost, the phenomenon was rather short-lived. Could it have been an island or some other large obstacle in the way, breaking the water surface?

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  2. Can't calculate the reflection location because for that you would need to know the crystal layer height from the ground. A map that shows the line at which the reflection roughly has taken place in Kirkkonummi case is here:
    https://www.taivaanvahti.fi/observations/show/37437#3
    But this was spring melt time at its height and many fields were water logged with rather extensive ponds. So I guess the reflection can have taken place from pretty much anywhere, not just those blue areas in the map.

    And yes, there has been talks about the obstacles that break the water surface as one explanation for slicing. Maybe the two narrow slices could be due to two small ponds. But that offsetting from the solar vertical is one more thing that asks for explanation. This was by the way very windy day. Water surfaces could not have been calm.

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  3. That's really puzzling. Do you think it's a possibility that the source of reflection is not water, but a large shiny glass or metal surface? Then the wind would not be a problem. If it's an inclined surface, like the top of two greenhouses, or that of a large factory, would that not offset the sun's rays?

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    1. Well, on second thought this still cannot explain the offsetting. Hm.. no idea, to be honest.

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  4. Some kind of inclined surfaces must be the explanation for offsetting from the solar vertical. But what it is, who knows. In the Kirkkonummi obsevation the offset area is quite wide. Overall, the Kirkkonummi case was quite faint - and particularly the offset area - so it still could have been from water even if it was restless. Maybe the offsetting comes from a widened reflection.

    All these options and more have been discussed already in Taivaanvahti forum. Once from an airplane I looked at reflections from the ground, and true enough, there were plenty of bright reflections from buildings much outside the solar vertical. So it would seem buildings can be the culprit too.

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  5. Actually I checked the map, and found that in the line of reflection there are some buildings with huge inclined roofs made of some shiny material. But I don't know if these would be large enough for such a phenomenon. And anyway, we can now only make guesses.
    These are the buildings I meant:
    https://goo.gl/maps/Hq1MvrCtUKC2

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  6. Possibly not large reflecting surface is needed for those thin slices. After all, we see in the autumn short segments of pillars from outdoor lights in Ac or Sc virga.

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