SSAT Practice Test 13

Why the Sky Changes Color

¹Ask almost anyone why the sky is blue and you are likely to hear that it reflects the ocean. The explanation is tidy, popular and wrong. The sky is just as blue over the Sahara as over the Pacific, and it stays blue above the middle of continents thousands of miles from any sea.

²Sunlight looks white, but it is a mixture of every color, each with its own wavelength. When this light enters the atmosphere, it meets molecules of nitrogen and oxygen that are far smaller than any of those wavelengths. Particles that tiny scatter short wavelengths much more strongly than long ones. In 1871 the British physicist Lord Rayleigh worked out the rule: blue light is scattered several times more strongly than red.

³At midday, then, blue light is bounced from molecule to molecule across the whole sky and reaches our eyes from every direction, while most of the other colors travel straight on. Violet light is scattered even more than blue, so why isn't the sky violet? Partly because sunlight contains less violet than blue to begin with, and partly because our eyes are much less sensitive to it.

⁴At sunset the same process produces the opposite effect. When the sun is low, its light must pass through far more air to reach us. Along that long path, most of the blue is scattered out of the beam before it arrives, and what remains is weighted toward orange and red. Dust or smoke in the air can deepen the colors further.

⁵Clouds follow a different rule. Their water droplets are much larger than the wavelengths of visible light, and particles that size scatter all colors about equally. Mixed together, those colors look white, which is why a cloud glows pale at noon and turns pink at dusk only when the light reaching it has already lost its blue.