What Causes the Sky to Look Blue?

One of the eternal questions long thought unanswerable is what causes the sky to look blue. Modern science and the understanding of light waves have provided an answer, although many false answers are still popular. What causes the sky to look blue is the result of electromagnetic scattering, using the light from the sun and the particles of Earth’s atmosphere.

To understand what causes the sky to look blue you must understand how light waves work. A light wave is a vibrating field of energy that travels in waves. The distance from one wave to the next is called the wavelength, and determines the color of the light. Light from the sun contains all different colors; all blending together to form a white light that travels in a straight line until it bumps into anything. Depending on the object it bumps into, the light can be broken into different lengths of light waves. The shortest light waves correspond to blue and violet colors, which also have the most contained energy and the highest frequency.

When a light wave hits a gas particle, such as those that make up the atmosphere surrounding earth, the particle can absorb some of the energy of the wave. Whatever energy the particle absorbs, it radiates or reflects. Because they have the highest frequency, blue light waves are much more often absorbed than their slower, less energetic companions.

Rayleigh scattering, a process named after the English scientist who discovered it, describes what causes the sky to look blue. Because of microscopic textures in the surfaces of the atmosphere’s particles, the blue absorbed light doesn’t just radiate or reflect in one direction, it shoots off in all sorts of directions. As you look at the sky, you are seeing the scattered light as it bounces all around the atmosphere.

Rayleigh scattering also describes why the sky seems to lighten in color toward the horizon. Because you are looking at sky much farther away, the light waves are passing through more air to get to you. The farther away you look, the less concentrated the scattering becomes, causing lighter blue shades near the edge of the horizon.

If you are wondering why, at sunrise or sunset, the sky turns red and orange, this is also a scattering process. When the sun is near the horizon, it must pass through much more atmosphere. As it does, the longer light waves that pass through thinner layers of the atmosphere are the only waves not to be completely diffused by the scattering. The longer light waves make up orange and red colors, so that is the light that becomes visible at sunrise and sunset.

Popular false explanations suggest that the sky is blue because the light reflects off of the blue oceans. In fact, it works the other way around. What causes the ocean to look blue is the same thing that causes the sky to look blue. The ocean is reflecting the color of the sky, so if the sky is scattered blue light, particles in the ocean will receive and scatter mostly blue light.