Moving Ripples in a sundog

On March 26, 2017, I could observe this phenomenon for the second time. The first time I observed it in Munich about 20 years ago, when within a few minutes two boundles of “moving ripples” crossed a left-hand sundog. At that time I did not know what I was seeing. I learned it thereafter, also the name of the phenomenon, that it has been observed several times until then and that it may be related to acustic waves. Later, the video of the “extermination” of a sundog by a rocket launch became well known. But I did not see this phenomen again till March 26, 2017.

It is just a “must” for me to photograph with my pocket camera every halo I see mostly only to get the time mark of its beginning and/or end for the record. On that day I was several times on my balcony to check for halos. The sky had only contrail-cirrus (now officially termed Ci-homomutatus), but no halos. But once I discovered a faint sundog it may have been the only halo-active contrail-cirrus group of that day. I observed the sundog coming and going with the respective cirrus couds resp. the standing sundog against the moving clouds. The sundog was faint the whole time and all but remarcable. But then I surprisingly realized that there were some odd dark strokes crossing part of the sundog diagonally. Remembering the moving ripples, I immediately zoomed in. I could record this phenomenon in some pictures. It did last only about 30 seconds, but visually the dark stripes were much more evident than the photographs suggest.

The exceptionality of this observation was that the otherwise “moving” ripples were in fact “standing”: They moved with the cloud through the sundog. This can be seen very nicely on the photographs: the ripples seem to be a fixed structure of the cloud. But they were only visible in the area of the sundog. Outside this area the ripple pattern did not show up: the dark stripes were not there! Clearly recognizable is also the fact that each ripple began weak and increased its intensity towards a maximum in the centre of the ripple area, and to vanish at the other border of the ripple area.

Remarcable was also the fact that the ripples showed up only in a part of the cloud resp. the sundog area. For me it remains a mystery why (only) a small part of the cloud was “trapped” in these “acustic waves”…

Author: Christoph Gerber, Heidelberg (BW), Germany

Link to the topic: Collection of all known observations

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