[MUSIC]
Clouds represent the largest uncertainty in the climate forecast, and
they're really the hardest part of the whole thing.
The biggest difference between different climate models, and
how much they forecast a change in climate, is pretty much due
to different ways in which different climate models treat clouds.
So clouds have two possible ways of impacting the energy budget of the earth.
One is by visible light, and the other is by infrared light.
The way clouds affect the visible light is by a process known as scattering.
So we've talked before about light coming in and
being absorbed by, say, a droplet of water.
Which means tha the energy from the light has been translated into kinetic
energy of the molecules in the water, which is the temperature of the water.
So you absorb the light you warm up.
And then, the warmer droplet will shine energy out in the infrared.
So we've seen this before.
But another thing that can happen is that the visible light comes in and
it's got this electrical dipole.
And it can actually induce an electrical dipole in the droplet.
So the droplet of water can be a little bit plus one side,
and little bit minus on the other side.
And flipping back and forth in resonance with the light that's coming in.
And then, this oscillating dipole, which is chained to the frequency of
the light that comes in, can create light that then shines out.
And it's because the frequency is the same as the light that comes in,
what leaves is visible light, as well.
So this is called scattering.
And scattering works best if the size of the cloud droplet is comparable,
to the size of the wavelength lambda of the light.
And it turns out that most cloud droplets are actually much bigger,
than the wavelength of visible light.
So the smaller the cloud droplet is, the better it can scatter.
If cloud droplets are really big, that's when you see dark clouds.
They're dark because they're absorbing the light, and
if you see dark clouds you've got to take your umbrella, because it might rain.
Whereas if the clouds are really bright, that means that the cloud droplets
are small, and that you know instinctively, based on your experience,
that bright clouds don't rain as much, and this is why.