Double Rainbow and reflected Rainbow

When I wanted to go to work in the morning of February 24, 2010, I noticed a colourful rainbow forming during a short local rainshower. At first the weather had been fine that morning, and ist was almost calm, but then some rain clouds came up from the southwest and started to cover the sky. The sun was shining brightly at that hour (about one hour after sunrise) and made the rainbow shine in especially bright colours. So I hurried to get my camera and started to make some photographs. While I was taking pictures, I was astonished to see a third bow forming, which intersected with the secondary bow. At the first moment I thought that there was something wrong with my eyes, but I could see it also when I looked at the pictures already taken on the camera display.

I was excited, although I could not make head or tail of it – I had never seen such a thing before. Later, as I searched the world wide web for an explanation, I learned that the phenomenon must have been caused by a reflection. But the reason for the phenomenon was still not clear, as there was nothing between me and the rainbow that could have reflected any light.
Only much later I could solve the mystery: The reflection was caused by the Hallwilersee (Lake Hallwil), which was at about 3 kms behind the position where I had taken the pictures that day. The lake cannot be seen from the place where I saw the rainbow, as there is a hill between the lake and that place. So I did not take this possibility into consideration at first. The position and elevation of the sun, my position towards the lake and the distance from the lake fitted perfectly that morning to form this rare phenomenon for a few moments.


Author: Matthias Frei, Dürrenäsch, Canton Aargau, Switzerland