I searched through the archive of past columns and didn’t find this one answered anywhere. I also searched through the message board archives and didn’t find it there, either. So, somebody please tell me – why are clouds white? I mean, they’re composed of water droplets, right? Raindrops are clear, so why not clouds?
The same question, I assume, could be asked about steam…
Regards,
Barry
You want them to work, don’t you?
Opps, sorry…thought this was the vulgar joke thread. 
Anyhoo, I imagine clouds are white just like snow is white. Individually, they are clear, but grouped together, give the appearance of being white. Just a W.A.G.
They are aspects of a different time/space continuum. See Larry Niven’s “On a Foggy Night.”
The better answer is that, though one droplet might be clear, each refracts light just slightly, and when you have millions of them in a chaotic pattern, the light is no longer clear.
Think of glass marbles. You can see light through one, but when you get a bucket of them, you can’t.
Actually, reading that story is what originally made me think of the question. 
My bucket of glass marbles may not look completely transparent, but neither does it appear white. Perhaps water and glass simply refract light in different ways?
Love the username, BTW!
Barry
Serious answer: Multiple scattering that’s pretty even across wavelengths. Glass is clear, too, but if you look through a lot of panes you’ll find that fresnel reflection throws most of the light back at you (and in other directions). Snow is white for the same reason. And ocean whitecaps (bubbles in clear seawater).
Have a look at M. Minnaert’s classic book The Nature of Light and Color in the Open Air, which is still in print (although not by Dover anymore, so it costs more these days).
…and polar bear fur, apparently.
What does “white” mean in the first place?
White paint is made of tiny transparent crystals suspended in transparent resin.
White quartz sand is made of clear bits.
White paper is transparent cellulose fibers and transparent clay crystals.
Take a clear block of ice and shave it into powder, and the powder is white.
Get some black plastic and abrade it with sandpaper, and it leaves white marks. Grind up some dark blue glass into extremely fine powder and it turns white (but turns blue again when wetted with acetone or other index-matched liquid which stops reflections from particle surfaces).
EVERYTHING that’s white is just a fine-grained light scatterer. That’s what the word “white” really means.
Actually it is because the ever-present phlogiston in the air which has provided nucleation sites to the water vapor is white, in and of itself. I’m still working on how the phlogiston turns red during combustion, but I think it’s realted to a doppler shift of some sort…