Why Do We See Colors When We Look at the Sky?

Light from the sun looks white, but it actually holds every color of the rainbow. When this light hits Earth's air, the air acts like a sieve that sorts the colors.

Imagine shining a flashlight through a glass of water with a pinch of milk. The water looks blue because tiny water and milk particles catch the short blue light rays and scatter them in all directions. The sky works the same way. The air is full of tiny molecules. These molecules are much better at catching blue light because blue waves are short and snappy. They bounce around everywhere, so when you look up, your eyes catch this scattered blue light from all over the sky. This is called Rayleigh scattering.

What about when the sun goes down? At sunset, the sun is low on the horizon. The light has to travel through a much thicker layer of air to reach you. The blue light gets scattered away completely on the long journey. The red and orange light waves are longer and stronger. They plow through the air like a truck through a light snowfall, while the blue light gets stuck. So, you see the leftover red and orange colors. It is not magic; it is just physics doing its job. The air filters the light like a colored lens.

The sky is blue because the air "catches" the blue parts of sunlight and sends them to your eyes.

So, the sky changes color because the distance light travels through the air changes which colors get scattered or pass through. It is a daily show put on by sunlight and air.