And it was raining and I was getting a bit wet.

But as I was wondering along, I just cited idly thinking about rain and in particular.

Why is that?

Raindrops are kind of that big rather than that big.

I guess it's sort of starts by what sizes raindrops actually are, and in particular fact that they're that they're not actually all the same size.

There is a distribution of raindrops, eyes and in fact, the sum of the early work on this was done by a guy called Wilson Bentley.

It was a fascinating guy.

He was kind of, ah, autodidact.

He taught himself all the science he did.

He was fascinated by science.

Hey was a farmer from Vermont beginning of the 20th century, and he was really interested in things that fell out of the sky.

So, for example, he made sort of the definitive set of photographs of snowflakes and was really the person who showed that snowflake several different shapes.

But one of the other things he looked at was rain, and he measured with a rather elegant experiment the sizes off the drops that four out of the sky in different types of rainfall.

So he had this very neat experiment.

He took a tray full of flour, so it's fairly shallow layer of flour in the tray.

Just put it out in the rain for a couple of seconds, pulled it back in again and sew.

The raindrops will kind of fall in and soak some of the flower.

He then wait for the little pellets that kind of formed out of that to dry and then pick them out of the flower.

And then the sizes of the pellets told him about the size of the rain drops like a ratio.

Or were they the same?

Like I think, sort of coincidentally, they turn out to be.

So he did the proper thing.

So you do know the scientific experiment is you.

Then calibrate your results.

So he actually took drops of known size and dropped them, burn varying heights into flour and found out what sizes of pellets they made.

And he found that basically the size of the pellets corresponded almost exactly to the size of the drop that fell in in the first place.

So just by measuring the sort of size distribution of the pellets, he was able to find out about the size distribution of Green.

I actually, he's paid for here.

So here's his paper, where he explains his experiment and so on.

But he actually has photographs, or so one of his experiments showing what the raindrops looks like And you could see again this sort of ranges of different sizes, some little drop, some big drops.

But he also found that sometimes the size distribution was more uniform.

Sometimes they were bigger drop.

Sometimes there were smaller drops, so he basically found that that is a rule of thumb.

The heavier the rain was, the bigger the drops are.

But beyond that, he quite found quite a variety of different outcomes.

He was indeed the pioneer in this, and people then went on to make more quantitative measurements and sort of started findings of empirical rule.

So here's a paper game.

One of the definitive papers on this, which is that the only one page long by Marcia on Palmer in 1948 on what they found is that with different rates of rainfall.

So this is the sort of the number of drops versus drop size.

So what they found is that there are lots of little drops and then many fewer bigger drops.

In fact, this is ah, log scale up here.

So this is actually, you know, this is 100 times as many drops this size.

Is there all this size?

So it's This is actually because it comes out as a straight line on this scale, that means is actually an exponential distribution.

There are lots and lots of little ones, and not very many big ones.

It's always follows the same kind of distribution, but the sort of calibration off it depends on how heavy the rainforest.

In heavy rain, you get bigger drops.

There's something very beautiful about, like, just human studying rain.

It is, although it actually it turns out it has applications to So for example, one of the issues about radio wave propagation.

So if you're trying to broadcast radio off, got radar, it's affected by raindrops, and it depends on the size, distribution of the raindrops and actually the shapes of the raindrops as well.

So there was actually a sort of a practical application as well.

Is it just being an interesting thing to find out about the world?

So now we need thio sort of get to the bottom of why raindrops are the size they are and why they grow to a certain size and no bigger, which means we kind of need to go back and look at the physics a bit more about how raindrops grow in the first place.

Okay, so you have a cloud and a raindrop will start to form and really so this is a sort of a cloud of water vapor, and it's going to start condensing, so to start condensing, typically you need something which sort of seeds the condensation.

So it's thought that just a tiny little grain of dust or something is there in the cloud which water?

Then we'll start to settle on so you start forming.

A drop on that drop will grow.

They'll beam or more water will grow onto that drop, and also drops will crash into each other so they'll grow both by sort of accreting stuff from the vapor, but also by smashing into each other and going into bigger drops that way, and as they start to grow, they start out.

Was completely round on the physics as to why they're completely round is to do is surface tension.

The surface tension sort of pulls the thing into a into a sphere.

What happens?

Maybe I should draw a picture for this under your picture.

Let's have a picture.

If you got a water molecule, it's in the middle of the water.

It's surrounded by lots of other water molecules, and they're all pulling it towards them.

And so basically, there's no net force on our water molecule in the middle here.

But if you get near the surface, you're a water molecule.

You here, then there's more molecules below it than there are above it.

And that means, is that sort of imbalance force.

That's more things, pulling it downwards thin, the while pulling it outwards.

So if you can imagine trying to take an individual water molecule and pull it up to the surface that in here you can move it around how you like.

But if you get close to the surface, the other water molecules are trying to pull it back down again.

so you actually have to do some work to get it up in the other surface, you have until fight quite hard to get it to the surface.

That actually means that energy stored in the water molecules close to the surface.

And so the Maur surface area you have the Maur energy potential energy there is stored up there on nature.

Being kind of lazy prefers to have the lowest energy state.

And so, therefore, the lowest energy state for any collection of water is the one that minimizes the amount of energy stored near the surface, which means you basically want to minimize the surface area of the object on for any given blob of water.

The shape which minimizes the surface energy energy is a sphere, And so that's why small amounts of water will arrange themselves into spheres because surface tension makes that the lowest energy solution.

Unless so, that's how it ends up being distributed.

Just that mean we need a seed like a as a grain of dust or something for every raindrop.

I think there has to be, yes, because the raindrops have to know where to form.

So there's something special about that place.

Which of which the simplest thing is just a tiny little going to dust, pretty microscopic in size.

But there's just something up there which makes it seed at that particular point.

Okay, so you made your drop.

It starts out being spherical, but because of the surface tension effects is they're starting to fall.

And that means that basically, from the raindrops perspective, the air is kind of rushing up past it.

And at that point, we have to start worrying about aerodynamics.

Now we made a whole video about lift and aerodynamics, and what have you.

And perhaps the only lesson from that is that aerodynamics is really rather complicated on this is even more complicated because it's not like a wing on it, something like that, which is a fixed shape.

This is a raindrop, which means that as the air rushes past, it can rearrange its shape.

So not only do you have to worry about the flow of air, but you have to worry about what it's doing to the shape of the object.

I got like a wing made of jelly.

Exactly.

And not surprisingly, you know you've got quite a lot of air pressure and things happening.

It will deform and you'll end up with the thing no longer being spherical.

That's where we get our beautiful raindrop shape from sadly not no.

So you think so?

Because, you know raindrop, nice air foil shape.

Turns out that's not the shape you end up with.

And although the physics is really rather complicated, there's quite a simple way to think about it, which is as the thing is, falling, this kind of air pressure from its perspective, it's being pushed from below.

And so what?

The simplest thing that starts happening.

It just starts to flatten the bottom.

So now the air keeps flowing around it.

But now it's got a kind of got off our flat bottom to it.

So it's like a hamburger bun, Jake.

It ends up being so we're still round on the top.

So then that process sort of continues.

The pressure below keeps going, and what happens is it actually starts.

The bottom starts in, isn't even flat.

It starts to turn com que on.

At that point, air kind of gets trapped inside the thing on your whole raindrop actually blows up like a little balloon with kind of a ring of water at the bottom of it.

And they're not surprisingly, that surface tension is not enough to hold a thing together.

And then it bursts into 1000 little fragments.

So you call actually have a big rain drop because it's unstable.

In that way, the air pressure will kind of blow it up and make it explode.

It almost acting like a parachute.

It is, Yeah, I think so.

Let me show you a picture.

He's really are fascinating Who thinks I hear some experiments, and so that is still an active area of research is ah, on article in Nature Physics, which is one of the more prestigious journals in physics from 2009.

And so here's the picture.

He's our large raindrop starting to fool.

He gets flatter as it falls.

Then you could see at some point it actually starts to inflate.

This little bag forms above it.

The surface tension.

Hold it together for a little while.

Then it bursts, and then actually the ring that's left behind fragments as well, and you end up with many, many tiny fragments.

So at what point in the journey.

Oh, he is.

Is it still up in the cloud here has a fallen 10 miles like health.

Long does this take It depends how big how quickly it grows.

Break.

So if you have a raindrop that goes big very quickly these happens very early on.

If you have the raindrops which stay small, this process will never happen.

It'll actually just they sort of rain drop, shaped and keep falling So it really is when the things go above a millimeter also in size That's when thes processes start to happen.

When they shatter into lots of small fragments, what happens to those fragments?

So, in principle, if there was time, the whole process will start over again, and it was thought that they would start growing again.

The interesting thing and the point of this paper, actually, is that the distribution of these fragments sizes is exactly the same as the distribution that these guys found what was actually hitting the ground.

So the hypothesis they come up with in this paper is that actually what happens is that that this process only happens once that the rain large raindrop forms.

It shatters into this distribution of smaller fragments.

And those are things which fall on your head at the ground level on that, then the size distribution of those then matches the distribution that we see in actual raindrops.

So those the fragments shrapnel that lands on us doesn't go through the balloon process again.

It looks like it.

And they argue, just on the basis of the time scales that the reason time that you can figure out the amount of time it takes a raindrop to fall to the ground and then sort of do some calculations as to how quickly it should grow.

And the answer is, it doesn't just doesn't have time to go through that cycle again.

Mike.

It feels like this ballooning and shrapnel process would happen so quickly and cloud seems so high up in the sky that you'd feel like this would go through.

Lots of it orations, But But no, I think the issue is that in a cloud, you have a lot of water vapor, which means that actually, things can happen very quickly.

By the time that thing's going that this sort of free four phase there isn't that much water around.

So the chances of things bumping into each other, much lower the chances of more water accreting onto your drop a much lower, so they just need some time for these processes that happen.

They haven't quite quickly in the clouds because it's just a lot of water there.

But by the time the things are falling, there's lots of gaps between the water, and so it doesn't happen again.

But I thought the shrapnel pieces would have the chance to go through the bullion process.

Or they too small for that now.

No.

So they're now back in this regime where the surface tension is the winning four, so that they will then turn back into these more or less spherical things.

So the raindrops which fall on your head, probably arm or less round everyone.

Professor Mike Merrifield her You just watching this video?

Well, he's an astronomer.

You can see even more of his videos here on 60 symbols or a special astronomy channel.

Deep Sky videos.