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. There is a common misconception that the sky is blue because of the reflection from the large water bodies on earth. The actual fact is that as the Sun emits white light - a composite of several colors - the Earth's atmospheric contents interact differently with these colors.

Our atmosphere is a largely gaseous mixture and other materials in which Nitrogen (78%) and Oxygen (21%) are the most prominent. Inert gases, water (in the form of vapor, droplets and ice crystals),traces of other gases and small particles -  such as dust, soot and pollen - make up the remaining 1% of our atmosphereAtmospheric composition varies depending on many factors such as location and weatherThe atmosphere is most dense closer to the Earths surface and it gradually thins out in the higher layers of the atmosphere. There is no sharp break between the atmosphere and space.

The reason why the sky is blue basically because of a phenomenon known as Rayleigh scattering. This is where the visible white light from the sun is however dispersed into the seven colors by the atmospheric contents  and the blue light is greatest benefactor of this phenomenon compared to the other colors during the day. In the early morning and evening, red and yellow light are the most scattered hence the sky becomes an orange hue.

In contrast, our moon which has no atmosphere has a black view of outer Space. There is basically no interference encountered by the Sun's rays as they travel from the Sun to the moon which means that someone standing on the moon's surface will see a black sky and a white Sun. 

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) are 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