"Don't go so fast," the Forecaster warned him. "I want you to think of the sky, first. It's a dead black, a hundred miles up. Now, at a hundred miles up, the air is so thin that there's little or no dust, but as you gradually come down and the air becomes denser, it begins to be able to buoy up some dust. Boys," he said, breaking off suddenly, "why does a stick float in water when it falls in air?" "Because water is denser than air?" guessed Ross. "Exactly. And why does a bar of iron sink through water and not through earth?" "Because the particles of earth won't move aside as easily as the particles of water, I suppose." "Not quite, but something that way. So, you see, as the air gets gradually denser it becomes gradually more able to support particles of dust, light ones at first, then heavier and heavier, until near the earth big pieces of dust can be carried in the air. You know how big some of them are when you happen to get a grain in your eye! Viscosity has a lot to do with it, too. "The light of the sun is a bluish-white, like some of the blue stars. White, as you remember from the rainbow, is just a mixture of all sorts of colors and the different colors are created by waves of light, some being shorter and others longer. A long wave, like the red, will pass around a tiny piece of dust, but a short wave, like the blue, will be stopped by it, and scattered, sometimes polarized, as it is called, or turned into one plane." "I don't think I quite see that," said Anton. "It's a little complicated," the Weather Man answered, "but maybe I can give you an idea of it. Suppose you were on a big steamboat in a choppy sea. As the steamer's length would extend over several of these waves, none of them would be big enough to make the vessel heave. If you were on that same choppy sea in a small canoe, you would be tossed in every direction. Now, if you think of the long red wave of light as a steamer and the blue as a canoe, you can see that in a ripple of small particles of dust the blue is going to be more affected than the red. In other words, the blue will be scattered. It will be diffused all over the sky and the light that comes through will be less blue." "Then I should think the sun would look red," said Anton. "It does," the Forecaster explained, "when there's a fog, which simply means, when there's more obstruction in the air. Sunlight is never white, as you know, it's yellow-white and the golden effect is due to dust. It's the same way at sunset. Then the rays of the sun which reach you pass through a larger amount of air, because you're looking at them from an angle, so they have to strike more grains of dust, and more of the blue rays are scattered. Then, too, when the sun, at sunset is, to you, shining obliquely on the atmosphere, it is passing through several layers of air and these bend the rays differently."