Why Is the Sky Blue? Quiz
Sunlight is a mix of colors
Sunlight looks white, but it's really a mix of all the colors of the rainbow. You can see them separate when light passes through a prism or a raindrop.
Light travels as waves, and each color has its own wavelength. Red light has the longest waves that we can see, and blue and violet have the shortest.
Seen from space, the Sun is white. It only looks yellow from the ground because our air changes the light on its way down to us.
Scattering
Air is made mostly of tiny nitrogen and oxygen molecules. When sunlight hits them, they bounce some of it off in every direction. This is called scattering, and short waves are scattered far more than long ones.
So the blue part of sunlight is scattered all across the sky, and when you look up in any direction you see blue light coming toward you. The effect is called Rayleigh scattering, after the British scientist Lord Rayleigh, who worked it out in the 1870s.
Violet scatters even more than blue, but the Sun gives out less violet, some is absorbed high in the air, and our eyes are much more sensitive to blue. Together, that makes the sky look blue.
Sunsets, clouds and other skies
When the Sun is low, at sunrise or sunset, its light crosses much more air to reach you. Most of the blue is scattered away on the way, leaving the reds and oranges.
Clouds look white because their water droplets are much bigger than air molecules and scatter all colors about equally. Thick clouds look gray because less light gets through them.
Without air there's nothing to scatter light, so the sky looks black from the Moon even in daytime. The higher you climb on Earth, the less air is above you and the darker blue the sky becomes. On Mars, fine dust makes the daytime sky a butterscotch color.