Why the Other Colors?
Though generally blue, the sky may be red, orange, green, in fact, almost any color. It all depends on how light waves enter the atmosphere and what they encounter on their way down.
In the lower atmosphere are concentrated clouds, dust and all kinds of particle material. These being larger than air molecules, they scatter the longer wavelengths of light. When the sun is near the horizon its rays enter the atmosphere at a slant, passing through much more of the particle-laden air. All wavelengths are scattered, and only the longest red rays penetrate at all. So we have the ruddy hues at dawn and sunset. The more dust or cloud particles in the air, the deeper is the color. The less dust, the bluer the sky, because the long light waves come right through to the earth without being deflected, while the short blue light waves we see are the ones being bounced about by air molecules in the sky. Therefore, on a clear, relatively dust-free day, our marvelous “roof” is blue.
wol.jw.org/g87 12/8 Sunsets Reflect the Glory of Their Creator