The Various Colors Of The Sky

A photo of a partly cloudy sky.

The open sky is typically blue because of how light interacts with the atmosphere. Sunlight reaches Earth with all of the colors of the rainbow in it, but to our eyes, it looks white. This light is a form of energy that travels in waves through space, with each color having a different wave length. The waves are much shorter in blue light than in red light, meaning that it scatters much more when it hits small particles floating around in the air.

When sunlight enters the atmosphere, it encounters minute bits of dust, droplets of water, and molecules of gases, such as nitrogen and oxygen. So, as a result, the light scatters in all directions. Blue light scatters more than any other color, since it has the shortest wavelength of all the visible colors. Hence, due to this higher scattering of blue light, we perceive the sky as being blue. This scattering of light is appropriately termed Rayleigh's scattering, since it was first explained by Lord Rayleigh.

Now, the color of the sky isn't always the same. The color of the sky may look lighter or darker, depending on how thick the atmosphere is and how much dust or pollution is in the air. When there is less dust on a clear day, the color of the sky is a deep blue since there's less to scatter the light. But on a hazy day, or near cities where there's more pollution, the sky might appear more pale or grayish.

The color of the sky also varies throughout the day. At times of morning and evening, when the sun is low on the horizon, sunlight has to travel through a greater amount of the atmosphere to reach your eyes. This means that there is more scattering, and the shorter blue wavelengths get scattered out, leaving the longer red and orange wavelengths to dominate. This is why sunrises and sunsets are often splashed with beautiful reds, oranges, and pinks.

But why doesn't the sky look violet, since violet light has even shorter wavelengths than blue? Well, our eyes are not as sensitive to violet light as they are to blue. Also, there's less violet light to start with in sunlight, and almost all of it gets absorbed by ozone in the upper atmosphere anyway. So, even though the scattering favors violet, we still only see a blue sky.

This is the scattering effect that allows the sky to look different from different places or at different times. If you are at high altitude—for example, on a mountain—there is less atmosphere above you, and the sky will appear darker since there is less scattering. When you are in an airplane, you may see the sky turn almost black above you because you are above most of the atmosphere and there is very little to scatter the light.

All of this is further complicated by clouds. Clouds are made up of droplets of water or ice crystals that are millimeters across. They scatter all colors about equally, so clouds usually appear white. Clouds appear gray during storms, however, because they are much thicker, leading to them absorbing most of the light.

In summary, the blue color—and any color really—of the sky is a result of different atmospheric conditions and the scattering of sunlight by particles in the Earth's atmosphere. This is a selective scattering and tends to favor short wavelengths, such as blue, rather than longer wavelengths, such as red. The shade varies with time of day, weather, and the location of observation. It is a very simple and interesting example of how physics and natural phenomena combine to bring us the world we have today.