There’s no punchline. There actually IS such a thing as a flat rainbow and it’s name is Circumhorizon Arc. It’s a summer optical effect and only appears in certain parts of the world. Like a rainbow, the colors are formed by ice crystals reflecting the light like prisms. But this rainbow bar (not really an arc) has nothing to do with rain but rather with the height of the sun which has to be high enough in the sky to allow for the parallel colors to appear at the horizon. Given all that, I was SUPER excited to spot my very first Circumhorizon Arc during last week’s heat wave! While capturing a stunning 22 degree halo (read more about that below), I noticed the rainbow bar way below it. The sky was very hazy but the bar was definitely there. Wow! Thank you, sky, for holding such wonders.
To spot the bar, look closely at the bottom of the image. The 22 degree halo also shown above is literally a ring around the sun. They are huge (way bigger than they look in photos) and not rare but often only parts of the circle can be viewed from earth. The 22 degrees refers to the angle of refraction when the ice crystals in a cloud form a hexagon.