Extremely red rainbow

This red rainbow appeared while the sun was setting and persisted even some minutes after sunset. At that time it was rather dusky already, and the glowing tops of the distant Alps appeared rather unreal. A short time later, a red rainbow appeared, showing an intense red colour which in this intensity I had never seen before. It showed its maximum intensity about 5 minutes after the calculative sunset, but the sun had already sunk behind a mountain some time before. After only a few minutes, rainbow and afterglow faded away simultaneously. The picture is a panorama made of 4 portrait frames with the single frames slightly underexposed, but not processed. The pictures are taken at ISO 800, shutter time 1/40 sec, f/4,5, and a polarizer was used.

At such low sun elevations, all short waved colours of the light are scattered away on the long way through the atmosphere, leaving only the long waved red light behind. This red light reaches the observer´s eyes as alpenglow and as a red rainbow. As, due to their altitude, the clouds (in this case altocumulus in about 3.000 metres) receive sunlight even longer than the ground, in rare cases a rainbow can even be visible after sunset.

Posted on July 25, 2012, in observations, rainbow and fogbow, twilight phenomena and tagged , , . Bookmark the permalink. 2 Comments.

  1. Once I have seen the red rainbow, maybe not as vivid colous as in the image here. The lower parts (left and right) of the bow, did rise up from the horizon – for a short time before bow disappeared…

  2. Hello — I’m curious about your thoughts on the possibility of a red rainbow from post-sunset gloam only; i.e. not from direct sunlight on high clouds, but the light of the sunset causing the rainbow. I know that a rainbow needs a point source of light, but some sunsets have relatively concentrated glows (not as concentrated as the solar disc, but still limited to a few degrees.)

    I’m asking because I thought I may have seen such a rainbow. I have detailed the sighting here: https://www.reddit.com/r/atoptics/comments/c5tfkv/question_possible_to_have_rainbow_from_sunset/

    Based on the cloud level, the nearby hills, and the apparent location of the rainbow, it seemed like the only explanation was a post-sunset rainbow without direct light on the clouds/moisture.

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