Yes, yes, I know, a rainbow
is caused by the sun
, and is not a sunbow. Shut up
There exists a large waterfall in southeastern Kentucky. Why you would be in southeastern Kentucky is not relevant here. What is relevant is that the waterfall is in a somewhat deep canyon, though I can't imagine canyon is quite the word. Regardless, there is almost perpetually a fine layer of mist that blankets this particular waterfall. It's 125 feet wide and about 60 feet deep. The fall itself is named Cumberland Falls, aptly I might add, since it resides comfortably in the Cumberland River. The town nearest it is Corbin, KY, I believe. What's at all interesting about this somewhat isolated, misty waterfall in the middle of a rather mundane valley is the interesting property of the waterfall's mist when the (earth's) moon is full. Due to the canyon blocking ambient light, and the fine layer of mist, a semicircular arc displaying the visible color spectrum is visible. I've seen it. It's cool. They don't call it a rainbow.
It turns out that this phenomenon is rarer than one might otherwise expect. It seems that there is only one location in the western hemisphere where this 'moonbow' can be seen during every unobstructed full moon. Yup, you guessed it, Cumberland Falls.
Southeastern Kentucky, being otherwise rather devoid of gimmicky tourist traps, really went all out with this one. I can't say for sure that they coined the term 'moonbow', but let's just say I wouldn't wager much against it. They have gift shops, billboards, fliers, websites, radio ads, and imported migrant Italian circus clowns advertising this blessed event. So there you go.
If you ever find yourself wandering around southeastern Kentucky (or northeastern Tennessee or western North Carolina, for that matter!) and thinking to yourself "Gee, self, I'm bored, it's 1:00AM, and there's a full moon out. I wish I could find an awe-inspiring, interesting outdoor natural phenomenon, within driving distance and not terribly inaccessible, unique to this hemisphere..." you now have the perfect answer to your query. It's the (drum roll please) U.S.'s finest (and only) Moonbow.
(and I am not making this up)