OK, let's get one thing straight first: We probably can't establish the "most rare" weather phenomenon, because if it's so rare, how can we know it's even happened yet? Maybe the rarest weather phenomenon happened when the dinosaurs ruled the earth. Maybe the rarest weather phenomenon made the dinosaurs extinct! Maybe humans did see Earth's rarest weather phenomenon, and because we saw it even just one time -- or heck, even two or three times -- we just assumed it wasn't that rare. So, high fives all around for getting out of answering the question!
Probably not going to cut it, huh? While we might not be able to objectively answer the absolute rarest weather phenomenon, we can at least explore a few weather occurrences that can legitimately count as pretty freaking out of the ordinary. And besides, why stop at one rare weather phenomenon when we can talk about tons of different rare weather phenomena? So let's take a look at a few contenders and start with every film student's favorite: raining frogs.
Yup, it's no joke. Amphibians can and do rain from the sky, and it happens when frogs (or fish or whatever small waterborne animal you wish) are caught in a waterspout (the hydrologic equivalent of a terrestrial tornado). The frogs are picked up and unceremoniously dropped off on land, assuming the waterspout ventures to shore [source: Thompson]. But we've tricked you: While raining frogs is a little hard to believe, it's not as rare as a weather phenomenon can get.
Venturing on to more unusual events, let's talk about triple rainbows. While double rainbows may awe viral video subjects, they're not that unusual. (And all it really involves is sunlight hitting raindrops and reflecting off in a couple of angles.) Triple rainbows, though, are a big deal. That's because -- for one -- you can only see two of the rainbows with the naked eye; the third occurs behind you and is obscured by the sunlight (which is also reflecting the light). Triple rainbows are so rare to spot that no convincing photographic evidence even emerged until 2011 [source: Byrd].
A rare weather event also occurred in the Grand Canyon in 2014, when the entire basin of the massive geological monument was fogged in. Here's what happened: As the ground cooled after a hot day and cold, humid air rolled in, low stratus clouds filled the canyon from the ground up. The result? The 277-mile (446-kilometer) long, 18-mile (29-kilometer) wide and 1-mile (1.7-kilometer) deep canyon was completely filled with thick, foggy clouds, which sightseers could easily look down upon [source: Winter].
So are these the rarest weather phenomena? Who knows? But they're rare enough that we can confidently say you probably won't experience them in your lifetime.