Rainbows can normally only be seen if the sun is low, as otherwise you have to look down to see them. At sunset, a rainbow can form a semicircle. This picture was taken from my house in the late afternoon, when the primary rainbow is actually below the horizon. Note that the secondary one has the colours reversed.
In contrast, the photograph below was taken as the sun was about to set. Note that the rainbow almost meets the horizon at right angles. The red sky seems to have caused the rainbow to be predominantly red.
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