Reinhard Nitze observed at Barsinghausen (Niedersachsen, Germany) on the morning of July 7, 2007 a strange rainbow produced by a passing shower. Owing to the low solar elevation it had a predominantly reddish colour. When he saw the photos he remarked on the strange step change in brightness, colour saturation, radius and width near the left-hand base of the bow (photo with unsharp mask). The change is marked on the photo by an arrow and is not an artefact because it is present on other images (1 2 3). The upper part of bow is considerably brighter and more strongly coloured although blues are weak and violet is completely absent. This is not easy to explain. A cloud might be shadowing the weaker part of the bow but that does not explain the width or radius change. Smaller drop sizes in the lower region might be responsible and could be obscuring rays from larger raindrops further away. Effects of reflected light bows can be ruled out because no water was nearby. Somewhat similar unusual rainbows were imaged by the Japanese observer Yuji Ayatsuka.
Thursday, August 30, 2007
Strange Rainbow
Reinhard Nitze observed at Barsinghausen (Niedersachsen, Germany) on the morning of July 7, 2007 a strange rainbow produced by a passing shower. Owing to the low solar elevation it had a predominantly reddish colour. When he saw the photos he remarked on the strange step change in brightness, colour saturation, radius and width near the left-hand base of the bow (photo with unsharp mask). The change is marked on the photo by an arrow and is not an artefact because it is present on other images (1 2 3). The upper part of bow is considerably brighter and more strongly coloured although blues are weak and violet is completely absent. This is not easy to explain. A cloud might be shadowing the weaker part of the bow but that does not explain the width or radius change. Smaller drop sizes in the lower region might be responsible and could be obscuring rays from larger raindrops further away. Effects of reflected light bows can be ruled out because no water was nearby. Somewhat similar unusual rainbows were imaged by the Japanese observer Yuji Ayatsuka.
I can imagine some kind of inversion in the lower parts, where the rainbow is distorted. It would be strange to have smaller drops on the lower part.
ReplyDeleteThe other idea is some faint reflection (maybe on plants' leaves).
But in a few words, this rainbow is a really strange one!!!
Interesting photo! My guess is that there are two different rainbows here. One is the upper one from a storm with relatively large raindrops closeby, and the lower one is wider and smaller radius from a drizzling storm in the distance. That would explain why the low wide bow is cut-off at the top - that would be where the cloud base of the distant storm would be.
ReplyDeleteWhy the upper bow in the close rainfall would be cut-off there too - perhaps it wasn't, but the lower bow is just superimposed on it. I can't really tell from the photo if both bows stop at the intersection, but I think only the lower bow stops there, simply because the distant rainfall appears at a lower elevation.
I could and very well might be wrong, ofcourse. But I think this is the most logical explanation in this case.
Another (part of the) explanation perhaps could be that non-spherical raindrops produce these remarkable bow. Surface tension forces keep small raindrops spherical but as they fall large drops are flattened by air resistance. (It has even been observed that thunder caused droplets to oscillate between flattened and elongated spheroids causing the rainbow to shiver....)
ReplyDeleteThe suggestion of Noli is interesting, and could be a good explanation: it was early in the morning, the sun low in the sky in about ENE, and the rainbow in about WSW, in the direction of the northern fringe of the central-German hill areas (I looked up in an atlas). The synoptic weather situation: a showery cyclonic-westerly airflow. Therefore, it's possibly indeed, the lower atmosphere was affected by the nearby hills, and hence an inversion caused the strange distortion at the foot of the rainbow. So, possibly, this is an example of atmospheric refraction, visible in a rainbow!
ReplyDeleteFrank Nieuwenhuys,
Den Haag, Netherlands.
Refraction is an interesting idea, but it would be an extreme form of refraction in this case, to cause a jump that large at such high elevation... atmospheric refraction is usually bound to the astronomical horizon and only a few arcminutes in vertical extend at best. Also, it wouldn't explain why the first supernumerary of the lower part is more widely spaced (visible in USM version of photo). I really think there are two separate droplet size distributions responsible for this shift. Oblate droplets would distort the rainbow too, but the change would be gradual and not as abrupt.
ReplyDeleteOK, atmospheric refraction seems to be unlikely, unless there was an extreme situation.
ReplyDeleteDifference in droplet size (and even shape) seems to be more likely.
Altough ruled out in the text, as to me, there is still a possibility of reflection: perhaps not on water, but on another material, glass for instance.
Still, the strange misty appearance of the foot of the rainbow is mysterious ....
Frank
The explanation with the different shower strips pleased me very good. The weather situation was a gathering rain front. Whereas it was raining already on my position before the front was generally there. At the same time was the sky directly over me was nearly blue and it was to seen only some streaks of rain stripes.
ReplyDeleteHere is still a picture of the complete "work of art".
Reinhard Nitze
"The explanation with the different shower strips pleased me very good."
ReplyDeleteReinhard, there can't be two different rainbows seen by one person at the same moment. (Only in case one has twin suns above his home planet :-) )
No matter how far the raindrops are - distant shower or a near one or both - you have only one sun, with one antisolar point that makes the center of your rainbow. ( see: http://atoptics.co.uk/rainbows/primcone.htm )
I think Harald was wrong with this part of his idea.
After studying the pictures again,
ReplyDeleteI think, the rainbow arose relatively far away from Reinhard's observation point. The strange 'misty' appearance of the 'foot' is caused by patches of small and thin Stratocumulus clouds, being in front of the rainbow. About 10 kilometers to the W and SW of Barsinghausen is a small river (according to may atlas). Perhaps that small river caused the strange 'branch' near the foot.
Frank
maybe this is caused by the different rate of frequence of the waves in the atmosphere, the red color is the higher of then, and blue is lowest, if you look from this point of view is more easy explaind this.
ReplyDelete