Thursday, August 30, 2007
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.