Why the sky is blue

Have you ever wondered why the sky changes colors? Most of us would describe the sky as blue yet in most afternoon comes it turns to different shades of red. Most people assume that the sky is blue because of the reflection from the large water bodies on earth, but that is not actually the case. The atmosphere is a mixture of gas molecules and other materials surrounding the earth. It mainly consists of the gases nitrogen (78%) and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common elements in the atmosphere, along with traces of other gases. There are also multiple small solid particles, like dust, soot, ashes, pollen, and salt from the oceans.

The composition of the atmosphere varies, depending on your location, weather, and many other variables. There may be more water in the air after a rainstorm, or near the ocean for example. Volcanoes can put large amounts of dust particles high into the atmosphere. Pollution can add different gases or dust and soot.

The atmosphere is dense (thickest) at the bottom, closer to the Earths surface. It gradually thins out as you go higher into the atmosphere. There is no sharp break between the atmosphere and space. The sky is blue basically because of a phenomenon known as Rayleigh scattering. Light is made of a spectrum of seven colors colors from red to violet. Our eyes perceive the visible white light from the sun, it is however dispersed into the seven colors by the air molecules. Earths' sky is blue which is quite different from the moons sky which is actually black since it doesn’t have an atmosphere.

Rainbow
how a rainbow is made.

The light color having a short wavelength blue along with indigo and violet of approximately  450 to 495 nanometers. These colors (blue light) is easily and widely scattered making the earths atmosphere appear as blue.

Rayleigh scatter
blue sky showing rayleigh scattering
blue sky showing rayleigh scattering

The sunsets and sunrise provide a spectacular view in the morning and afternoons. Most people are not able to explain why that is happening. The reason for the red and yellow light seen in the morning and evening is because light has propagated a long distance having most of the small wavelength light almost completely scattered and ones with larger wavelength have remained. In summary, the green blue and violet have been filtered out. The red, yellow and orange colors are the ones left, thus the therapeutic, spectacular view.

Red sky
Red sky sunset

So now with the new found knowledge, we should be able to better explain why the sky looks the way it does.

References and resources
  • https://doi.org/10.1093/carcin/18.4.811
  • https://www.mcgill.ca/oss/article/environment-general-science-you-asked/why-sky-blue-or-better-yet-why-ocean-blue
  • https://www.metoffice.gov.uk/weather/learn-about/weather/optical-effects/why-is-the-sky-blue