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You’ve probably heard the phrase “it’s raining cats and dogs outside.” And you probably also know better than to take that literally. But sometimes, animals do fall from the sky.

There have been rare but regular reports of this phenomenon going all the way back to ancient Rome. Sure, some of the historical accounts are probably exaggerated, but there are enough well-documented cases in this century that we know that animal rain is definitely a thing. And meteorologists think they know why.

Well, mostly. If you read Pliny the Elder’s stories of frogs and fish falling from the heavens, you might be understandably skeptical. But in 2005, thousands of frogs rained down on a small town in Serbia.

Then, in 2009, clouds of tadpoles fell in Japan. And in 2010, small fish that were very much still alive pelted a town in Australia’s. Northern Territory for several days.

That was the third time that town had experienced animal downpours since 1970. And similar fishy weather events have been reported from Wales, India, the Philippines,. Honduras, Sri Lanka, Ethiopia, and Mexico in just the last twenty years.

Other places have described storms of birds. Also spiders. No!

The question is, where are all these animals coming from? And how are they getting up in the air? In some cases, there’s a perfectly ordinary explanation that freaked-out observers just aren’t aware of.

When spider rain fell in Australia three years ago, for example, of course it was Australia, it was probably just a case of ballooning, where spider babies use their silk to float long distances on air currents. It’s perfectly natural, if slightly terrifying when the entire landscape is suddenly coated in a thick layer of cobwebs. And the thousands of blackbirds that fell out of the sky in Arkansas and Louisiana in early 2011 probably died from crashing into things when they were startled by the fireworks on New Year’s Eve.

It’s those other animal rain incidents—the aquatic ones—that are much harder to explain. In the past, people thought maybe migrating birds were dropping their prey en masse. No one had any idea why they might do this, mind you.

But why else would frogs fall from the sky? There’s no such thing as a frog with wings. These days, most scientists and meteorologists think these animals come from waterspouts: tornadoes over open water, like oceans or large lakes.

They’re most common in tropical oceans, but they’ve been seen all over the world, and they can form over water even when conditions are relatively calm. All it takes is enough warmth and humidity for clouds to condense and touch down. The waterspouts that form in fair weather tend to be small, weak, and short-lived.

But sometimes, waterspouts form when land tornadoes move out to sea. These are less common, but they’re also much larger and more dangerous. Regular tornadoes have been known to carry lightweight debris like paper as far as 320 kilometers, and heavier objects like metal signs for 80 kilometers.

I mean, Dorothy made it all the way to Oz. So while waterspouts aren’t usually as strong, the idea that they might suck up things, transport them some distance, and then drop them in a new location does make sense. And if they do, those things could include small fish or frogs.

Several of the living hail events in recent years coincided with storms and strong winds, which also fits the waterspouts theory. The fact that the animals are aquatic also makes sense. And occasional reports of animal rain in places far from water could be explained by land-based tornadoes or other strong updrafts instead.

The thing is, no one has actually ever seen this happen. It isn’t exactly easy to observe a violent storm over water, so it could just be that meteorologists who study waterspouts aren’t on high alert for airborne fish. Or, since these “amphibious rain” events are so rare, maybe they haven’t been in the right place at the right time.

So even though the waterspouts idea fits really well, we haven’t been able to confirm it. Maybe if we did, we could also explain another mysterious detail: why they usually involve just one animal or even one species. It’s frogs, or fish, not a random mix of nearby aquatic life.

Some scientists think that that’s because objects of similar size and weight fall out together as storm forces die down. Others find that answer less than satisfying, since lakes and oceans contain lots of different animals that are fairly similarly-sized. Another possibility is that the spouts only pick things up when they’re densely clustered.

But until we actually see animals getting pulled out of the water by spouts, no one can really say what limits the storm’s catch. So, if fish or frogs start falling from the sky and plopping onto the sidewalk, don’t panic — it’s probably not a sign of the end of times. It’s just a really weird weather phenomenon with a perfectly reasonable scientific explanation.

Probably. But even if it’s not raining frogs, the science of weather is fascinating. Which is why I wanted to test my understanding of atmospheric pressure systems with this Brilliant quiz.

If you want to join me, Brilliant set up a link so you can test your knowledge too. Click on the link in the description or go to brilliant.org/SciShowAtmospheric in a new tab. I’m taking a quiz.

So I first got excited about meteorology when I was in college because there was a cute girl who was really into meteorology. This is how my… my interest in…. It wasn’t the weather that did it.

So I’m taking my quiz here. Is moist air more likely to exert a higher or lower pressure than dry air of the same temperature? Oh.

Hmm…. I would say higher would be my guess, but now I’m guessing. I was wrong.

Moist air is lighter than dry air. Huh. I guess because water is lighter than nitrogen.

That makes sense. Once it’s… it doesn’t seem like that would be the case. I hadn’t thought about that, which is why, like, clouds rise up.

Duh. Get your act together, Hank. One of my favorite twitter accounts is the National Oceanic Atmospheric Administration satellite's twitter.

Very good satellite data coming out these days. And I love it but I feel a little bit like I don’t actually know what’s happening even though this is my job. And I’m I’m going through this and realising that I indeed need to hone my skills to be a person who better understands what’s actually going on.

And you can do that too at Brilliant.org . The First 77 viewers to sign up at Brilliant.org/Scishow will get 20% off their annual premium subscription and you will support SciShow. So thanks! [ ♪ Outro ].