As sunlight passes through the atmosphere it gets refracted or scattered by air molecules; light nearer the blue end of the spectrum tends to get scattered more than other colors, giving a blue hue to the sky.
At sunrise and sunset, light traveling through longer distances in the atmosphere becomes less scattered, allowing more red and violet wavelengths to reach our eyes.
Sunlight
Isaac Newton demonstrated with a prism that sunlight contains all visible colors. But for light to reach us on Earth, it must first travel through our atmosphere, where its interaction with atmospheric molecules determines its color. Light with shorter wavelengths (like blues and violets) scatter more easily, giving rise to what is known as Rayleigh scattering that gives our skies their characteristic hue – no matter whether we are terrestrially situated or orbiting space! This process also accounts for why skies appear blue no matter your position on the planet!
However, blue-tinged atmosphere also produces other effects which contribute to variations in sky color. One such effect is due to nitrogen and oxygen molecules being smaller than visible light wavelengths; when sunlight strikes them it gets scattered all directions but more efficiently with shorter wavelengths, as described by Lord Rayleigh in 1871.
Air molecules don’t completely cancel out the effects of shorter wavelengths, however. This is because our eyes contain three types of color-detecting cones that respond differently to wavelengths; red and green cones respond more sensitively than blue cones despite light from the sun being evenly scattered across the sky, leading to it appearing bluer due to being scattered more efficiently than red wavelengths.
At dawn and dusk, this effect is most striking as sunlight has had more time to travel through the atmosphere and air molecules have had more opportunities to scatter away blue light, leaving reds and oranges behind to give the sky its characteristic hues of whitishness. While similar processes take place during the day due to bulk attenuation – resulting in less blue sky colors around noontime and sunset.
Atmosphere
Sunlight traveling through Earth’s atmosphere is scattered by gases and particles of its composition; this gives the sky its color. Since oxygen and nitrogen molecules in air are much smaller than visible light wavelengths, they scatter blue light more than other colors.
Rayleigh Scattering occurs when red and violet wavelengths that have longer wavelengths are not as scattered and pass directly through the atmosphere, so when seen from ground level the Sun looks predominantly blue.
Water molecules absorb red and orange wavelengths emitted by the Sun, reflecting back only lighter blue wavelengths towards us.
The blueness of the sky can also be affected by humidity, pollution and dust levels in the atmosphere. Countries closer to the equator with increased levels of moisture pollution or dust will have less of a blue sky than Australia for instance – similarly for planets that possess different atmospheres.
At higher altitudes such as on an airplane or mountaintop, atmospheric particles which scatter sunlight will have less of an effect. Conversely, during sunrise or sunset viewing sessions, when light must travel further through the atmosphere it may produce redder hues in the sky.
Weather that is clear and dry can create the illusion that the sky appears more vibrant, while haze in the air makes light waves more clearly defined, decreasing their likelihood to absorb or scatter.
Dust and Water Droplets
As sunlight passes through the atmosphere, it comes into contact with tiny particles of dust and water vapour in the air that scatter and reflect its light in many different directions, with wavelengths of blue being absorbed more than other colours – the result being why skies appear blue. When the sun is higher in the sky, blue refraction is strongest directly overhead and gradually fades as it hits ground at horizon, due to having had to travel further through atmosphere before reaching ground surface where more light has been scattered by dust or droplets on ground level.
As the sun becomes lower in the sky, more of its light must pass through more atmosphere before reaching you – this means less blues are being scattered effectively, allowing more reds and yellows through to reach you without competition from blue hues. This phenomenon explains why sunsets often feature vibrant red hues; further away from it at nighttime the sky gradually palers.
Rayleigh scattering, named for Lord Rayleigh who established its mathematical formula in 1871, describes this phenomenon. According to this theory, its intensity varies as the fourth power of wavelength being scattered; shorter wavelengths (such as blue) being more easily dispersed than longer ones ( such as violet).
This also explains why open water appears blue – water molecules absorb blue light more effectively than their deeper ocean counterparts which contain much smaller particles with minimal effect on its appearance.
Clouds and fog consist of tiny droplets distributed randomly throughout, creating the impression of an overall white color. However, coronae, glories, and ghostly fogbows seen in some clouds are caused by groups of droplets that have come together forming clusters for more concentrated appearances.
Imagine living on the Moon without any atmosphere to provide contrast; astronauts on Blue Origin suborbital flights have reported similar experiences to me.
Rayleigh Scattering
As sunlight travels through the atmosphere, it encounters many different gases and particles which can diffuse its light or cause it to be absorbed and lose energy, either through Rayleigh scattering or absorption processes. Rayleigh scattering is the most prevalent form of light scattering seen during daylight hours – its cause being blue-tinged skies!
Rayleigh scattering occurs when air molecules scatter light particles evenly in all directions, but do so more strongly for shorter wavelengths such as blue and violet than longer ones like red. As blue light is scattered more strongly than other colors, it passes more easily through our atmosphere without interference from molecules and reaches our eyes more directly.
Water molecules absorb longer wavelengths efficiently, which gives open seas and oceans their blue hue. This effect is due to Rayleigh and Mie scattering; more so as the sea approaches the horizon where more atmospheric particles interact with its molecules.
Air above us doesn’t always appear blue; rather, its hue depends on atmospheric density and composition. At higher elevations, for instance, air is thinner and spread out more readily, meaning blue light has less time to reach our eyes – this explains why the sky at these elevations often appears bluish-violet; at ground level however it often appears paler or even whiter in hue.
The color of the sky depends heavily on dust, pollutants and water vapor levels in the atmosphere. These elements affect how light is scattered across wavelengths as well as saturation of blue portions of spectrum; when there are too many pollutants or water vapor present they will reduce how much blue light reaches our eyes resulting in duller skies that appear brown rather than bright blue – this phenomenon being more prominent in urban settings as opposed to rural ones with cleaner air. Furthermore, red and orange portions of spectrum may not reach us directly due to further travel through atmospheric layers before reaching us at this distance from where they pass before becoming visible at this location.