Some places on Earth get over 10,000 times more rain than others, which is a huge difference between the rainiest and the driest, but is it possible that rain all over the globe, however often it happens, follows the same patterns? Or does it play by its own unique set of rules, depending on if it'sfalling in Australia, Antarctica, or the Gobi desert?
A team of European physicists investigated whether rainstorms follow similar rules regarding how often they dump a certain amount of water, and what they found could finally settle a debate physicists have been waging for decades: rain marches to the beat of its own (soggy) drum.
When physicists are faced with a problem that no one has ever solved before, often they'll find that the solution uses exactly the same mathematics as a completely unrelated problem.
Nature repeats patterns all over the place, so sometimes Schrödinger’s equation will pop up in fish behaviour, or fluid dynamics will pop up in the middle of electricity. These interconnections are everywhere, and it’s often a physicist’s job to find them.
So when a group of German and Spanish physicists started looking at rainfall records from a collection of places around the world, they expected to be able to describe the rain, regardless of where it landed, in terms that they already knew.
Of course, we know that the tropics get more rain than the desert - that’s what makes them the tropics and the desert. But these researchers weren’t interested in that kind of difference. They wanted to know if rain around the world has the same relationship between its volume and its frequency.
We can make up numbers about a couple of the places they looked at to see what that would mean.
If the site in Papua New Guinea usually has a week between small rainstorms and a month between large ones, and the site in Niger has two weeks between small storms and two months between large ones, then both have the same relationship between their small and large storms: large storms happen four(ish) times less often.
All you have to do is multiply the amount of time Papua New Guinea waits by two and you get the amount of time Niger waits.
If you could multiply by Papua New Guinea by a different number and get Germany, and then multiply by a different number and get Australia, then the relationship between large and small storms would be the same in all of those places, and it would probably be the same around the world. Once you knew the relationship for anywhere, you’d know it for everywhere.
The researchers almost found this kind of similarity. The so-called critical exponents, which are related to the relationship between the large and small storms, were just a little too different between locations.
Rain, it seems, isn’t universal - at least not to physicists, and they aren’t exactly sure why. It could be that just one or two of the seven sites they tested were messing up the results, or it could be for a deeper reason that they haven’t thought of yet.
It’s always fun to learn that something simple has hidden connections beneath the surface that stretch into unexpected territory. But sometimes rain is just rain.
Thie study has been published in Physical Review E.