Why Clouds are White--Part 2

  Heading north from Olympia Friday morning, I almost made a U-turn under these beautiful cloudy skies when I so much more gray than white. Maybe clouds really weren't actually white after all and I'd be off the hook for the final ta-dah installment of this long-running blog topic. But I kept driving. 
   I was off to see the wizards, the wonderful wizards in the University of Washington's Atmospheric Sciences Department in Seattle who were willing to help answer my question about clouds.
    The man behind the curtain this morning was a graduate student (below) who knew the atmospheric science side of clouds as well as the physics side. Though one graduate student had already provided me with a perfectly excellent "classical physics" explanation of why clouds are white, I had been flirting with the ideas presented in another style of  physics called quantum electrodynamics, or QED. 
What better way to discuss the whiteness of clouds than with a whiteboard and a brilliant grad student! 
   Don't be afraid of QED! "Quanta" refers to the units of energy, the photons, that make up light. You may have learned that sunlight is a form of energy that moves in waves. This is true, but the waves themselves are made up of other things--the photons, also referred to as particles, or packets, of energy. "Electro" refers to the electrons (negatively charged units of energy) present within atoms of matter. "Dynamics" refers to forces that produce change in a system. So, QED is the study of how light and matter interact or do what many physicists refer to as "their little dance."
   The classical physics explanation--and the one you read pretty much everywhere--tells us that clouds are white because they scatter all the visible wavelengths present in sunlight equally. When white light reaches a cloud, a cloud droplet may separate into its component colors (red, orange, yellow, green, blue, indigo, violet) but another cloud droplet will recombine the colors or refract (bend) them in a slightly different direction. This happens over and over and over within the cloud so that the net result is white. None of the wavelengths is favored so the light retains its original color--white--when it reaches our eyes.
    To understand the QED explanation, let's start with a blue sky. Outside the cloud, in the clear atmosphere, the nitrogen in the air does something different that water does to light. Nitrogen is preferential; it scatters the blue wavelengths more efficiently and therefore our skies are blue. That is because the electrons in a molecule of nitrogen (two atoms of nitrogen) match the energy level of certain short wavelengths (the ones that appear blue to us).

This is why the sky is blue. N stands for nitrogen. The rest is kind of self-explanatory.
    These electrons absorb the energy of the photons in this wavelength and re-emit it as blue. This particular dance of light and matter, danced on the stage of our atmosphere, turns the sky blue. 
Ignore the Bald Eagle. Just look at all this nitrogen! 
    This makes sense to me (it is true as far as I know) but for some reason, I needed to find out what, to a photon, was the difference between a molecule of nitrogen and a molecule of water. Why does light behave differently?   
 Within a cloud we have cloud droplets of all different sizes (10-100 microns) and air (nitrogen, oxygen, and other molecules) in between them. Within the cloud droplets are many very active water molecules, which are oscillating, rotating, vibrating, and zinging around. Within the molecules, we have electrons occupying a cloud-shaped spaces around the centers (nuclei) of the hydrogen and oxygen atoms in the molecule. The electrons do not "sit" in a fixed position the way planets do in their orbits around sun. Au contraire! Scientists can only offer probabilities of where they might within the atom at any given time. Trying to pin down the location of an electron is like trying to pin down the location of an ocean wave.
This letter 'e,' symbol of the electron. Unlike actual electrons, this e  is visible, super-sized, cemented into one position, and I know exactly where it is at all times: The Olympic Sculpture Park in Seattle.
Water molecules--H20--contain electrons that exist at certain energy levels.  This graph shows the different levels.  The Y axis shows the energy levels, and the X axis something I wasn't allowed to know about yet. 
  When pure white sunlight reaches the cloud, the different wavelengths of visible light start the dance. They might dance with the nitrogen or oxygen molecules between the cloud droplets or enter a droplet itself. The photons do not "bounce" off the outside of the droplet. 
   At this point, it is easier to treat the light as particles/photons instead of as waves. 
   Within the cloud droplet, the photons might encounter the electron or it might not. No one knows for sure. And why not? There is much empty space inside an atom and no one can actually see electrons and photons or the dance they are doing at the speed of light--186,282 some miles per second. 
   If a photon of a certain energy level encounters an electron, and the electron has an available energy level that is the same as the photon, the electron will absorb the photon, "jump" to a higher energy level, then fall back to its ground state or even a lower energy state. As it falls back, it releases energy. That energy is re-emitted in the form of a photon. If that photon is a yellow wavelength photon, yellow light will be emitted. If the photon is a green wavelength photon, green light. 
     No one can predict what any one photon will do. This, as it turns out, really doesn't matter. What matters is that when wavelengths in visible light that encounters certain-sized water droplets will be absorbed and re-emitted multiple times before it reaches our eyes. While a certain wavelength photon might be absorbed and re-emitted more than another for a very short while, overall, no particular wavelength is absorbed and re-emitted more than another. A white cloud one way our eyes see the result of a bazillion encounters of light and water.  
Gray is the new white.
       It is, unfortunately, more complicated than this, but I am losing sight of the clouds themselves as I explore the mysteries of their subatomic life. Seeing clouds through quantum electrodynamics is like listening to Bach's Cello Suites through the individual hairs on the cello bow. Each hair is important, and its particular role fascinating, and it's all so beautiful really but, at this micro level, I am unable to hear the music.  
   

White Clouds Half-Time Show


Photo by J. Blevens
Poetry, by Pablo Neruda

And it was at that age...poetry arrived
in search of me. I don't know, I don't know where
it came from, from winter or a river.
I don't know how or when,
no, they were not voices, they were not 
words, not silence,
but from a street it called me,
from the branches of night,
abruptly from the others, 
among raging fires
or returning alone,
there it was, without a face,
and it touched me.

I didn't know what to say, my mouth
had no way
with names,
my eyes were blind.
Something knocked in my soul,
fever or forgotten wings,
and I made my own way,
deciphering
that fire,
and I wrote the first, faint line,
faint, without substance, pure
nonsense,
pure wisdom
of someone who knows nothing;
and suddenly I saw
the heavens
unfastened
and open,
planets,
palpitating plantations,
the darkness perforated,
riddled
with arrows, fire, and flowers,
the overpowering night, the universe.

And I, tiny being,
drunk with the great starry 
void,
likeness, image of
mystery,
felt myself a pure part
of the abyss,
I wheeled with the starts.
My heart broke loose with the wind.

from I Explain a Few Things: Selected Poems, by Pablo Neruda. Translated by Alastair Reid. 

Why Clouds are White--Part 1.98

Richard P. Feynman (1918-1988)
     I've been hanging out for the past week with Nobel-Prize wining physicist Richard Feynman, trying to get him to explain why clouds are white. I now have six of his books and have read around in five and am listening to another on CD. Luckily, Feynman is good company. He is very entertaining and has much to say about photons and electrons; much of what he has to say takes the form of diagrams featuring many squiggly little lines representing things I fear there are no words to explain.
Some of Feynman's books and a partial reflection of light in/on glass ( topic of one of his lectures). 
   On Sunday, I talked to my mother in law who, by some inexplicable crazy coincidence, has also been reading Feynman's books. We talked about space-time and rainbows and how exciting it is to discover physics and Feynman over fifty. Neither of us pretending to understand any of it.
   On Tuesday morning, a box arrived in the mail (below). Two books from my mother in law! One is the lively autobiography of Feynman the funny guy, the other a graphic novel with Feynman and his ideas presented in helpful and colorful cartoons. I was beginning to understand some physics and QED theory. (But not enough so that I don't bother to tell you that QED stands for quantum electrodynamics.)
Explaining clouds and light requires much thinking outside the box.
   Feynman doesn't write specifically about light and cloud interactions and what is happening inside the cloud droplet, water molecules, and atoms of hydrogen and oxygen. But he gets close! In The Feynman Lecture #32 on light scattering, he asks directly, "why do ever see the clouds?" Unfortunately, I am not smart enough to understand the answer which is three long paragraphs long and ends with these squiggly lines that represent polarized light which may or may not have something to do with clouds. I just can't tell yet.
This is from The Feynman Lectures and is as close as the physicist gets to clouds.

 On Tuesday night, I was pretty exhausted from my Feynman-a-Palooza, so I found my way on to the Internet (again) to try to understand the underpinnings of quantum electrodynamics as it relates to white clouds. I was zinging around reading about physical optics, polarized light, dipoles, 'n' such, and I found this:
From A Primer on Particle Sizing Static Laser Light Scattering, 
by Paul A. Webb of the  Micrometerics Instrument Corporation.
  No wonder I am having a hard time! Just look at what is happening in and around just one particle in two dimensions. Light is scattering every which way by reflection and refraction and diffraction. But...what kind of particle does that circle represent? A cloud droplet or a water molecule? And what about those rays? How does a ray compare to a beam? Are they on the same scale as the round particle? Is this just a cartoon? Can something as small as a wavelength or a particle even be drawn? Or is this circle and these arrows symbols--like the number 4 not really being anything like 4. I wish I had taken more science classes so I wouldn't think so literally.
   Oh, I am struggling because I want to see the light and the cloud droplets interacting. I want to see the angles of reflection and refraction and diffraction inside the cloud. I want to see which electrons are excited by which wavelengths of white light in the white clouds. Someone draw me a photon dancing with an electron!
    I know it's a lot to ask. Too much perhaps. And maybe it doesn't matter, the way learning about the earliest stone-scratched letters of our alphabet doesn't matter to understanding the meaning of a sentence made up of those letters.
   But still...I am determined to offer something more than an equation, more than a diagram of squiggly lines moving along axes.
   What made me think I could do this in two parts?