Q: Why is the Sky Blue?

A: There is a quick and easy answer to this common question. The sky looks blue because earth's atmosphere scatters sunlight. The sky is not actually blue, it only looks that way. It does not always look blue and even when it does it's not quite the same color or the same brightness everywhere. To scatter is to strew about aimlessly or to disperse. The atmosphere does that to light, but it scatters some colors better than others Sunlight is actually all the colors of the rainbow mixed together. The visible spectrum is the colors red, yellow, green, blue, indigo, and violet. Sunlight reaches us in the form of waves and each color consists of waves of a slightly different frequency. Of visible light, red has the lowest frequency and violet has the highest frequency. The frequencies of the other colors fall in between. At the end of the 19th century an English scientist, Lord Rayleigh, discovered that small particles do not scatter light in the same way as large particles. Air molecules are extremely small and scatter the high frequency violet and blue better than the low frequency red. Scattered sunlight contains more violet and blue than red but we see blue better than violet so the sky looks blue. At least it looks blue most of the time. At sunrise and sunset the sky is red, orange, or yellow. Rayleigh scattering is responsible, but the light passes through more air before it reaches our eyes. By the time the sunlight reaches us, scattering has removed the high frequencies to make blue skies for people on the part of earth where the sun is higher in the sky. Red light remains because it is scattered less. Big droplets of water which collect around bits of dust in the air to form a cloud or fog are too big for Rayleigh scattering. They scatter all colors equally so the clouds they form look white. Smaller water droplets which collect around dust or pollutants in the air form a yellowish haze.. Rayleigh scattering essentially absorbs the incoming light, then re-radiates it in all directions. We then see this scattered light coming to us from all directions. The scattered light is not only blue. It contains small amounts of the other colors as well, so the sky is not just blue. It is sky blue.


Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical sciences. Send questions on any science topic to: Richard Brill Honolulu Community College 874 Dillingham Boulevard Honolulu, HI 96817 email : rickb@hcc.hawaii.edu

Blue Skies ©1995 Richard C. Brill