Treebeard's Stumper Answer
Black and White Clouds
I don't get to fly very often, so our Dunn Middle School flight to Washington D.C. last week was a real treat! I sat glued to the window trying to plot our course on my map, but I got lost coming home because the heartland of the country was covered with brilliant white clouds. They look like you could snuggle up in them like a giant white comforter. Our first day back at school was also our first winter rain of the season, and a reminder that those thick white clouds look very different from below. Water is colorless, so why do rain clouds look so dark from below, and so white from above?
Looking out my airplane window at bright white cloud
tops somewhere over Missouri. The clouds are breaking
up as we fly west away from the storm center.
A rainbow in the Santa Ynez Valley last week. Note how
dark the clouds are from below. The storm is starting to
clear out, and white cloud tops are visible in the breaks.
Water, air, and clouds really have no particular color of their own. They appear colored because of the way they scatter the sunlight that contains all the colors of the rainbow. The oceans and the atmosphere scatter blue light the most because of the small size of their molecules. The larger water droplets in clouds scatter all colors equally, which we perceive as white. Thick clouds look dark below because not much light gets through them. The larger water drops in rain clouds absorb as well as scatter light, so they look the darkest of all.
I have to be careful how I say this. Someone might also claim the red barn also has no particular color of its own; it only appears colored since it scatters red light and absorbs all the other colors. The point is that clouds (and water and snow and air) scatter all light. They have no pigment of their own to selectively scatter light of certain frequencies. At a molecular level, a pigment is a texture, and color-talk gets even more confusing. Sometimes science and ordinary language are at odds for awhile, and stumpers skip from science into philosophy.
My wife Julie is an artist. She immediately answered this stumper by saying that white and black are not colors. I remember reading this as a kid and being confused since there are white and black crayons in the box along with all the others. How can they not be colors?
I really wanted to answer this stumper by saying that white and black are the same color. That was too hard to explain in my short stumper space. I think Julie and I agree that black and white (and gray) are different from red and green and blue. They involve no particular frequency of light, but all frequencies mixed together.
I use Paint Shop Pro as my computer graphics program. It lets me select colors in the usual computer fashion by selecting red, green, and blue (RGB) values, but it's hard to find some colors (like brown) that way. PSP also lets me select colors more intuitively using a hue, saturation, and luminance (HSL) color scheme. Hue is what we usually call color -- red, green, blue, etc. Saturation is how much of the other colors are present. Luminance is how bright the color is. With this scheme, white and black and gray all have the same hue and saturation. They differ only in luminance or brightness. The grays are in-between.
In this HSL color scheme, white and black really are the same color. White is bright, it reflects all colors. Black is dark, it reflects no colors. Gray is in-between, and it reflects some of all colors. They have a lot in common. Clouds are thick with tiny water droplets and ice crystals, so of course they are brighter above and darker below since they reflect and absorb all light. But the color (or lack of color) is the same.
Here are some more examples where perceived color depends on texture rather than pigment, at least above the molecular/pigment level:
- Water from the faucets at school sometimes looks milky-white because of tiny air bubbles. It clears when I let it sit for a while.
- Ground-glass fittings on laboratory glassware look whitish, as does the pitted windshield on my old car. But it's only scratched up (clear) glass. Crunch glass with a hammer for the same effect.
- Water is clear, but we've all seen white-water rapids where the water looks white because of all the tiny bubbles that scatter sunlight.
- Ice is clear, but snow is white beause of the crystalline structure. (Deep ice looks bluish, like water and the sky, because it scatters blue light the most.)
- Dry sand looks lighter than wet sand because it scatters more sunlight.
- Soap suds look white.
- Egg white is the protein albumin. Stirring, cooking, or pickling egg white turns it into an opaque white solid. These mechanical processes change the molecular shape of the protein and effect how it reflects light.
- Wheat is brown, but ground flour looks white. (Or is it just bleached?)
- Whole milk is white because of very-tiny suspended fat globules. Non-fat milk looks different.
Here are a few related stumpers to think about:
- White clouds reflect all colors, and so do mirrors! What's the difference?
- Mix all your watercolors together and they turn muddy gray, not white. Why?
- Cloud rings around the Sun and Moon are white, and so are fogbows. I've seen fogbows on my way to school! What gives?
- If the Earth were covered with clouds, it would have a higher albedo and reflect more sunlight. Would the Earth stay cloudy forever?
Here are some links for further research on dark clouds:
- This quetion about dark clouds has come up before on the Web, e.g. at Scientific American: Ask the Experts, New Scientist Last-Word, and ScienceNet.
- the Physics Classroom from Glenbrook South High School has a tutorial on light and color, and here's another. There's lots more info if you search.
- There are several general Web sites about clouds, including the UtahLINK Online Cloud Guide, WW2010 from the University of Illinois Urbana-Champaign (UIUC), and the PSC Cloud Boutique.
- My In a Fog (15 Jan 99) and The Sky is Falling (3 March 00) stumpers are also about clouds.
- I made my own Web page about local Santa Barbara Weather (etc.) conditions, with current satellite images and forecasts from many sources. Study the HTML source code to make your own local weather page!
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