i was going to talk about a thing called
the brunt vaisala frequency
you're going to have to do a lot of
explaining a lot of explaining well
first thing to say about it is one of
the reason why i wanted to talk about it
is it actually has a symbol so getting
back to 60 symbols roots for some reason
this particular frequency is always
given the symbol capital n and actually
i've tried very hard to find out why
it's called capital n and i can't find
anyone who would explain why it's called
capital n it's just every textbook you
find it in it's always called capital n
you'll find out in the comments probably
here
all right what actually is it so
actually what got me to thinking about
this is i just the other day i was
thinking i knew you were coming to do
some filming i think what can i talk
about and i was just glancing up at the
sky and it was one of those days where
the clouds were in lines you know you
sometimes see very neatly arrayed lines
of clouds and i realized that actually
there's a really nice simple piece of
physics which explain why you sometimes
get the clouds in lines like that you
find it particularly if there's a
mountain downstream from the mountain if
you've got air blowing over the mountain
you'll find these lines of clouds there
so what's actually happening is so we
need to back up a little bit and do a
little bit of physics which is i need to
talk about buoyancy um so if you've got
a balloon
a helium filled balloon of course it
rises because it's less dense than the
air around it
and obviously the less dense something
is the faster it rises and obviously if
it's denser than air it'll sink and if
you've got it exactly right it's exactly
the same average density as the air
around it it'll just float wherever you
put it and if you move your head up
it'll stay there and if you move it down
it'll stay there so that's a thing
called neutral buoyancy
that the thing just basically stays
wherever you put it
and that that's what will happen if
you've got kind of a uniform density
atmosphere that you can take something
if it's floating there you move it up
it's still floating
if on the other hand you've got a
stratified atmosphere so actually the
density decreases as you go up as the
earth's atmosphere does then what will
happen is you've got something which is
neutrally buoyant at this point and
let's get rid of the balloon let's just
have a little blob of air there
if you were to displace it upwards
suddenly it's surrounded by air that's
less dense than it is
and so it'll sink again and it'll keep
sinking it'll actually sink through its
original point and then it'll find
itself in a region where it's less dense
than the stuff around it and now it'll
start to rise and so it will actually
oscillate backwards and forwards and the
frequency at which it oscillates
backwards and forwards is this thing
called the brunt visalus frequency and
the reason why that has to do with these
lines of clouds is that if you have uh
some air with some moisture in it if it
starts undergoing these oscillations
then as it rises it'll cool and suddenly
it'll cool to the point where clouds
condense and so you'll actually make
clouds and then when it sinks again
those clouds will then evaporate again
and the clouds will disappear so if
you've got air which is traveling along
and undergoing these oscillations you'll
get clouds then the clouds will
disappear then the clouds will appear
again then the clouds will disappear
again in the atmosphere the period of
this oscillation is something it's of
order 100 seconds typically if you look
at the how the the atmosphere's
stratified and use all the physical
parameters it works out about 100
seconds and typically air is traveling
at maybe 10 meters per second
normal kind of wind speed so
in each oscillation it will have
traveled about 10 meters per second over
100 seconds you travel a kilometer and
that's why the scale of these clouds is
typically a kilometer or so that you'll
get a cloud then a gap then a cloud and
there's typical spacing between them is
of order a kilometer so it's just
depositing a little cloud at the top of
its wave
it's actually well the same air will
form the cloud and then the cloud will
disappear again so you basically create
the cloud and then the cloud disappears
and the cloud appears so you're sort of
seeing those high peaks when it goes up
it forms the cloud and it's wherever
you're at at the top you'll get the
cloud and then it disappears again and
then reappears again
won't that oscillation fade away when it
eventually stop oscillating and just
sort of
stop like re like a like a pendulum
eventually runs out of puff if you just
look at the mass you actually figure out
what the mass of this oscillation is it
really is i think this thing called the
harmonic oscillator and the harmonic
oscillator will just keep oscillating
forever now obviously there are other
effects going on as well
so there are will be damping terms which
means that actually one of these waves
won't actually last forever but it
really can persist for many many
oscillations so actually you can have
this whole series of clouds being
created in that way so this frequency
comes up all over the place so it comes
up in oceans because you can have
exactly the same effects in oceans of
pockets of water rising and falling in
stars you can have little bits of the
star in the stellar atmosphere rising
and falling and they're all div driven
by the same frequency this by silent
frequency
and again something here again you can
see the net pull of the earth and the
sun is all nicely lined up which will
keep it in an orbit basically following
following the earth around there used to
be lots of science fiction stories about
l3 it used to be a popular place to put
the anti-earth or you know if you're an
evil genius that's where you put your
hidden layer