Name: The Size and Shape of Raindrops - Sixty Symbols
Uploaded: 2020-04-04T09:00:08.000Z
Duration: 11 min 41 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

That's how I knew you were coming in to do some filming, and I was wondering into work thinking about what to talk about.

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

Adverbs of degree

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

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.

He taught himself all the science he did.

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.

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.

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

So you do know the scientific experiment is you.

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.

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 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.

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.

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

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.

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.

Maybe I should draw a picture for this under your 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.

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.

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.

Because, you know raindrop, nice air foil shape.

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The Size and Shape of Raindrops - Sixty Symbols

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