so i knew you were coming in to do some
filming and i was wandering into work
thinking about what to talk about and it
was raining and i was getting a bit wet
but as i was wandering along i just
started idly thinking about rain
and in particular why is that raindrops
are kind of that big rather than that
big i guess it sort of starts by what
sizes raindrops actually are and in
particular fact
they're not actually all the same size
there is a distribution of raindrop size
and in fact the some of the early work
on this was done by a guy called wilson
bentley he was a fascinating guy he was
kind of a an auto died he taught himself
all the science he did he was fascinated
by science uh he 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 snowflakes
are all different shapes but one of the
other things he looked at was rain and
he measured with a rather elegant
experiment the sizes of the drops that
fall out of the sky in different types
of rainfall so he had this very neat
experiment he took a a tray full of
flour so a fairly shallow layer of flour
in a tray
just put it out in the rain for a couple
of seconds pulled it back in again and
so the raindrops would kind of fall in
and soak some of the flower
he'd 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 raindrops like
in in a ratio or were they the same like
i i think sort of coincidentally they
turned out to be so he he did the proper
thing so he's doing the scientific
experiments you then calibrate your
results so he actually took drops of
known size and dropped them from 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 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 rain i actually have his paper here
so here's his paper
where he explains his experiments and so
on but he actually has photographs of
some of his experiments showing what the
raindrops look like
and you can see again this sort of
ranges of different sizes some little
drops some big drops but he also found
that sometimes the the size distribution
was more uniform sometimes there were
bigger drops sometimes there were
smaller drops um so he basically found
that
the as 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 finding
sort of empirical rules so here's a
paper again one of the definitive papers
on this which is actually only one page
long by uh marshall and palmer in 1948
and 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 were lots of little drops and then
many fewer bigger drops and in fact this
is a log scale up here so this is
actually you know this is a hundred
times as many drops this size as there
are this size so this is actually
because this comes out as a straight
line on this scale that means it's
actually an exponential distribution
there are lots and lots of little ones
and not very many big ones it always
follows the same kind of distribution
but the sort of calibration of it
depends on how heavy the rainfall is
that in in heavier rain you get bigger
drops there's something very beautiful
about like just humans studying rain it
is although it actually it turns out it
has applications too so for example one
of the issues about radio wave
propagation so if you're trying to
broadcast radio or if you've 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 as it just being an interesting
thing to find out about the world so now
we need to 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
will start to settle on so you start
forming a drop
and that drop will grow there will be
more you know more water will 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
growing into bigger drops that way and
as they start to grow they start out as
completely round
and the physics as to why they're
completely round is to do with surface
tension the surface tension sort of
pulls the thing into us into a sphere
what happens maybe i should draw a
picture for this can i draw a picture
yeah let's have a picture if you've got
a water molecule if 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've got a water molecule here then
there's more molecules below it than
there are above it
and that means it's a sort of imbalance
force there's more things pulling it
downwards than while pulling it upwards
so if you can imagine trying to take an
individual water molecule and pull it up
to the surface in here you can move it
around how you like but as 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 near the surface you actually have
to fight quite hard to get it to the
surface
that actually means there's energy
stored
in the water molecules close to the
surface
and so the more surface area you have
the more energy potential energy there
is stored up there
and nature being kind of lazy prefers to
have the lowest energy state
and so therefore the lowest energy state
for any uh 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
and for any given blob of water the
shape which minimizes the surface 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
and that's so that's how it ends up
being distributed does that mean we need
a seed like 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 grain of dust
probably 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 because because of the surface
tension effects it's then 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
our whole video about lift and
aerodynamics and what have you and
perhaps the only lesson from that is
that aerodynamics is really rather
complicated and this is even more
complicated because it's not like a wing
or any something like that which is a
fixed shape this is a raindrop which
means that as the air rushes past it 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
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 a with the thing no longer being
spherical that's where we get our
beautiful raindrop shape from sadly not
though so you think so because you know
raindrop nice aerofoil shape turns out
that's not the shape you end up with
and although the physics is really
rather complicated there's a 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 the
simplest thing that starts to happen is
it just starts to flatten at the bottom
and so now the air keeps flying around
it but now it's kind of kind of got a
flat bottom to it so it's like a
hamburger bun shape it ends up being so
still round on the top but flat on the
bottom so then that process sort of
continues the pressure below keeps going
and what happens is it actually starts
to the bottom starts it isn't even flat
it starts to turn
concave
and at that point air kind of gets
trapped inside the thing and your whole
raindrop actually blows up like a little
balloon
with a with a kind of a ring of water at
the bottom of it and then not
surprisingly the surface tension's not
enough to hold the thing together and
then it bursts into a thousand little
fragments
so you can't actually have a big
raindrop because it's unstable in that
way the air pressure will kind of blow
it up and make it explode
it's almost acting like a parachute it
is yeah and so let me show you a picture
these really are fascinating
cute things so here's some experiments
and so this is still an active area of
research this is an uh an article in
nature physics which is one of the more
you know prestigious journals in physics
from 2009. and so here's the picture
here's our large raindrop starting to
fall it gets flatter as it falls then
you can see at some point it actually
starts to inflate
this little bag forms above it
the surface tension holds 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 are we is it still
up in the cloud here has it fallen 10
miles like what how long does this take
it depends how big how quickly it grows
big so if you have a raindrop that goes
big very quickly this happens very early
on if you have raindrops which stay
small this process will never happen
it'll actually just stay sort of
raindrop shaped and keep falling
so it really is when the things grow
above a millimeter or so in size that's
when this these 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 would start all over again and
it would start they would start growing
again the interesting thing and the
point of this paper actually is that the
distribution of these fragment sizes
is exactly the same
as the distribution that these guys
found of what was actually hitting the
ground
so the the hypothesis they come up with
this in this paper is that actually what
happens is that the this process only
happens once
that the rain large raindrop forms it
shatters into this distribution of
smaller fragments and those are the
things which fall on your head at the
ground level and that then the size
distribution of those then matches the
distribution that we see in actual
raindrops
so those the fragments the 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 there isn't 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 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 clouds seem so high up in the sky
that you'd feel like this would go
through lots of iterations 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 things are in
that's this sort of free-fall phase
there isn't that much water around so
the chances of things bumping into each
other are much lower the chances of more
water are creating onto your dropper
much lower so there just isn't time for
these processes to hand they happen
quite quickly in the clouds because
there'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 balloon process or are
they too small for that now no so
they're now back in this regime where
the the surface tension is the winning
force so that they will then turn back
into these more or less spherical things
so the raindrops which fall on your head
probably are more or less round
hey everyone professor mike merrifield
who you just watched in this video well
he's an astronomer you can see even more
of his videos here on 60 symbols or our
special astronomy channel deep sky
videos there are links on the screen and
in the description of course but mike's
also become the unofficial weatherman of
60 symbols he's done videos on lightning
and rainbows and clouds
there's a whole playlist again there's a
link on the screen and down in the
description