This is really cool – it’s an all-red rainbow! It was spotted by a fisherman in Finland over the weekend.
The red color happens the same way a normal rainbow happens, with the sun’s rays being reflected and refracted as they pass through water droplets. In this case, the short wavelength blues and greens were scattered in the lower atmosphere.
Thus, only reds and yellows remain, creating a red rainbow.
The fisherman who spotted it said that while seeing a rare rainbow is neat, it was a little unsettling and ominous so he went back home.