The thing that happens when its both sunny AND raining.
The water droplets in the air act as prisms refracting the ligth and splitting it into several colour bands visible as a bow of different colours in the air.
The bow is a ring but from ground level is only visible as an arc, if you are in the air or on top of a mountain it's possible to see all of it.
Also a fucked up kids tv show with Zippy (!Flippy), Bungle (!Braingrind) and George. see RainbowScript .
<EricCartman>Man, I hate rainbows</EricCartman>