As early as 1500, Leonardo da Vinci attempted to understand why the sky was blue. He kept a notebook that documented sunlight passing through wood smoke, and appears to have grasped light scattering phenomena.
The sky appears blue because shorter blue wavelengths are scattered more effectively by atmospheric particles than longer red wavelengths.
Sunlight
As sunlight passes through our atmosphere, its path is altered by gases and particles in the air. Some colors tend to be scattered more than others depending on their wavelength; blue light tends to get dispersed more than red light; thus accounting for why the sky tends to appear blue most of the time.
When we see a “blue sky”, our eyes actually detect all of the visible colors from a rainbow, yet our brain interprets these hues as blue – even though to the naked eye they appear white! Thus the sun emits a spectrum of all these vibrant hues which comprise the rainbow itself.
Skys are blue because when sunlight reaches our atmosphere, it has been scattered by nitrogen and oxygen molecules present. This occurs because the wavelength of light exceeds that of particles present; shorter wavelengths like blue and violet light scatter more easily than longer ones such as red or orange hues.
At daybreak, when the sun is high in the sky, its rays reach Earth at nearly vertical angles. As such, most of their light is absorbed by air molecules, with only blue and violet wavelengths reaching our eyes.
At sunset, the sun’s position in the sky decreases and its rays travel further through the atmosphere, meaning more red and orange wavelengths are absorbed while less of its blue/violet wavelengths pass through – leading to orange/red sky hues at dusk.
As the sun dwindles lower in the sky, its rays penetrate more obliquely into the atmosphere, absorbing more blue and violet wavelengths while less of its red and orange wavelengths are absorbed – creating an effect which produces skies of mixed blues and violets instead of a pure blue color – a phenomenon responsible for sunrise or sunset colors.
Atmosphere
As sunlight passes through our atmosphere it is scattered by tiny molecules of nitrogen and oxygen found within. Light at the blue end of the spectrum is more strongly scattered, giving our skies their signature color – this phenomenon is known as Rayleigh scattering after its discovery scientist.
All the colors of the rainbow get scattered to a certain degree, with blue being dispersed more than any of its peers, leading to our perception of mostly blue skies. It’s for this reason that darkness appears just before sunrise or sunset because more time has been allowed for light to pass through our atmosphere and scatter.
As the Sun moves lower in the sky, atmospheric molecules absorb more of the sunlight that hits them, and less blue rays are scattered to our eyes, leaving more reds and yellows for us to perceive. This phenomenon explains why planets with thicker atmospheres than Earth appear different.
Dust, aerosols and moisture in the air can alter the color of the sky in unexpected ways, desaturating blue light to produce more milky white hues. This occurs due to particles larger than light waves scattering all colors evenly (Mie scattering).
Have you noticed how the sky appears bluer over water versus land? This is due to shorter air paths over water which allows more of its blue light to reach you before reaching your eyes, leading to its diffusion by many more scattering processes before finally reaching you as light reaches you through your eyeballs. As such, sky appears brightest over ocean before gradually dimming towards horizon where atmospheric path length is 10X greater.
Clouds
Earth’s atmosphere is composed of gas molecules that scatter light. This causes longer wavelengths (such as reds and oranges ) to pass through easily while blue and violet wavelengths become increasingly scattered, deflecting them away from reaching our eyes and giving way to longer ones (reds and oranges ). Reflected light from the sky has an uneven distribution; open water appears blue because less light gets deflected away, thus allowing longer wavelengths to reach us directly. Sunrise and sunset make matters even worse as Sun travels further through our atmosphere thus scattering more blue wavelengths while red and yellows enter our consciousnesses.
At noon when the Sun is directly overhead, the sky appears brighter blue; while at nightfall when its path through the atmosphere must travel further to reach your eyes. This explains why daytime skies seem brighter blue.
Clouds contribute not only gases to the atmosphere but also sunlight. They scatter it evenly around, giving rise to various hues in the sky; but, generally speaking, it appears blue due to water droplets larger than wavelengths of light being scattered equally by all clouds. If clouds were smaller or made out of different materials however, their color would change accordingly.
If you look up at a clear sky during the day, it will appear whiter because sunlight must travel further through our atmosphere before reaching us. By the time it reaches you from its source, its energy has already been scattered many times before reaching its final destination.
Why doesn’t the sky appear black? Because our eyes contain three types of cones to detect color as well as monochromatic rods for monochrome perception, with blue and violet wavelengths scattering most heavily, creating the blue-tinged appearance. Indigo and violet wavelengths scatter more strongly, so they appear slightly less blue.
Sunlight on other planets does not reflect off of a sky-blue hue due to a lack of atmospheres – as with Moon light which has not been diffused by gases and dust particles, making its path not as blue.
Water
The sky appears blue due to all the water on Earth’s surface. Oceans and lakes reflect blue light from the Sun into our atmosphere, giving an illusion that appears as blue above us. Some animals also seem blue due to reflections in seas and lakes reflecting sunlight back onto them and stimulating color receptors within their eyes.
At noontime, the sky appears predominantly blue due to atmospheric gases scattering shorter wavelength (blue) light more than other colors. This phenomenon, called Rayleigh scattering, can be calculated using frequency times its fourth power. Early theorists James Tyndall and John Rayleigh suggested this was due to small particles such as dust or droplets of water circulating through the atmosphere, however these particles are far too insignificant to have any measurable effect.
Understanding our eyes helps explain why the sky is blue: our eyes contain three types of color-sensitive cells known as cones that respond differently to wavelengths of light; red cones react strongly to wavelengths of red light while not as strongly responding to blue or violet wavelengths – this explains why sky appears blue – longer wavelengths such as red and orange light are scattered less by the atmosphere, so they pass more clearly through it.
Why lakes and oceans appear blue is due to how water molecules react with sunlight; more blue parts of the spectrum are scattered by molecules than red or orange ones, thus producing clearer reflections at sunrise and sunset than during daytime hours. This explains why skies typically appear blue during the daytime hours before turning red with sunrise/sunset times.
Answering why the sky is blue requires multiple explanations; these three factors all combine to produce its signature color: sunlight contains wavelengths representing every hue in the rainbow; our atmosphere contains gaseous particles which scatter different wavelengths in various ways; and human eyes being sensitive to specific frequencies of wavelengths give this characteristic color to our atmosphere and sky. All three combine to give us our beautiful sky!