When people see two rainbows together, why do they often call them a "double rainbow" instead of "double rainbows"?

Possibly because one is a ghost of the other, less brilliant and more smoky. Probably just because it developed as part of the vernacular, like many deer, moose and fish but several coyotes, wolves and eels.

If you have one rainbow and double it, it is two. Double rainbows would be 4 or more..

We don’t say that a DNA molecule is “double helices” either. Nor do you do “double takes” when you see a handsome guy. By adding the modifier “double”, you’ve already captured the two-ness of the phenomenon; pluralizing the noun would be redundant. The only time you would make the noun plural is when you’re talking about more than a single instance of the phenomenon (e.g “I’ve seen 3 double rainbows in my life”).

Why are one goose but two geese, and one moose and two moose, or one deer and two deer, and one mouse but two mice? Who made these rules.

It is one rainbow that happens to have two visible bands: one at ~42 degrees from shadow your head makes on the ground and the other at ~ 51 degrees from the same point. If you extend your arm and make a fist, your fist width is about 10 degrees. Count up 4 -5 fist widths from the shadow of your head and make an arc. That is where the rainbow will be.

Maybe they are just In so much awe that they don’t realize what they’re saying.

It’s one single phenomenon with the second, fainter rainbow being produced by the refraction of light from the first. Note in this beautiful image how the fainter rainbow has its colors exactly reversed in comparison to the bright primary rainbow.

Because saying the word double enables you to leave the S off the second word. Like if two cats walked by you’d be like “WOAH! Did you see that double cat?” Not “hey look at those double cats”....then its just weird. THats how it is in my mind anyhow haha.

@deni That’s weird.

@ETpro, that’s better than my video linked above but not near as funny.

@Judi Indeed your tape is funnier than my answer. It’s extremely rare to witness a double rainbow that complete. I’ve only seem it once in 69 years on Earth. But it only means that conditions are idea for forming a very bright rainbow and a mirror image of it in the mist and rain that the sun is shining through.

@Judi: Aw, dude,

(Plus the additional pleasure of discovering the new verb, “to rainbow.”)

I had to think hard how to get that video in a “general” question.