As sunsets and sunrises approach, the sky often turns from blue to red as more atmospheric particles pass through as sunlight descends towards its point of origin.
As soon as sunlight enters our atmosphere, it is scattered all across it by atmospheric gases and particles, with blue light being more heavily scattered due to shorter wavelengths.
Light from the Sun
One reason the sky is blue stems from sunlight passing through our atmosphere to reach us; light’s hue depends on its wavelengths; shorter (bluer) wavelengths tend to be scattered by air molecules more readily than longer (redder) ones; when viewing sunlight through a telescope, blue wavelengths tend to be stronger scattered and so make up most of what reaches our eyes.
Rayleigh scattering, named for the scientist who first observed it in 1870, refers to gas molecules being smaller than visible wavelengths of light and colliding with light rays, thus either reflecting them back off them or absorbing them completely. With longer violet wavelengths tightly packed together they tend to reflect less while short blue wavelengths scatter much more; ultimately resulting in blue skies!
Not to be overlooked is that blue is not one solid hue – rather, it is composed of multiple tones of violet, greens, and blues that get mixed with red from sunlight. Our eyes contain three types of cone-sensitive cones as well as monochromatic rods to detect light; these sensors give our brains enough information about which hues of light we perceive.
Sunsets and sunrises complicate our perception of what appears as blue in the sky even further, due to how much of its energy must pass through more atmosphere than when overhead – this means more of its blue light is scattered away leaving more reds and yellows visible in their entirety.
That is why the sky tends to appear brightest above you and then gradually fade into white near the horizon. As atmospheric path length becomes significant, more light becomes scattered multiple times before finally being scattered with no polarization remaining at its source.
Rayleigh Scattering
Rayleigh scattering explains why the sky has its characteristic hue; it involves light being scattered by particles along its path, such as gas molecules in our atmosphere that are much smaller than wavelengths of visible light and can thus scatter it easily. As blue light is more strongly scattered than other colors of spectrum due to this process, this gives rise to its signature blue hue in our sky.
So that sunlight can reach Earth’s surface, it must pass through the atmosphere. Along its journey, however, sunlight will be scattered and redirected by various factors – including dust, pollution and the presence of water droplets – but mostly due to atmospheric gases like nitrogen and oxygen – usually smaller than wavelengths they are scattering thus making it easy to diffuse blue and violet light throughout space.
Rayleigh scattering gives the sky its characteristic hue at sunrise and sunset due to light having more time to interact with atmospheric conditions.
Light passing through an atmosphere can become polarized by molecules in its path, as electrons within these molecules are pushed back and forth against one another by photons of light. This results in changes to its electric field dependent upon which direction electrons are moving, leading to saturation changes with certain colors; hence why the sky appears blue from below while green when seen from a tree’s perspective.
The color of the sky can also change when seen from different perspectives. When moving towards the horizon, light will reflect off ocean waves or other bodies of water to recombine blue and violet light resulting in its characteristic hue; when moving away from it however, more land and cloud formations scatter it further away to produce lighter hues of blue hues.
Reflection
As the Sun rises and sets, its light travels through an immense atmosphere before reaching us – this journey follows Rayleigh’s Law where molecules scatter light indiscriminately according to wavelength. Blue wavelengths tend to be shorter than red ones so more of it gets scattered away, giving rise to blue shades appearing from our view – hence why the Sun appears bluer to us than other colours.
As light travels through the atmosphere, it reflects off any surface it encounters – water surfaces, mirrors and dust particles all reflect light; this reflection makes up what we perceive as sky when looking upward. Furthermore, any object with non-perfectly flat surfaces will appear slightly bluer.
The color of the sky depends on atmospheric gases as well as Earth’s surface itself, such as atmospheric CO2. For instance, Mars does not feature bright blue skies due to having much less atmosphere than Earth does, and Venus features dark gray hues because its atmosphere contains nearly 100 percent carbon dioxide compared to just 3 percent on Mars.
There may also be other factors contributing to the sky’s blue appearance, including clouds and dust haze. Clouds and dust contain particles larger than wavelengths associated with blue light; thus allowing it to scatter efficiently (Mie scattering).
Sky haze over mountainous regions could be caused by the reaction between vegetation terpenes and ozone to produce tiny aerosols of 200 nanometer diameter, perfect for scattering blue wavelengths while atmospheric ozone absorbs and darkens any other colors to make them appear black.
Sunlight fills all spectrums, but when low on the horizon it has already penetrated a large portion of our atmosphere and its brightness fades in intensity nearer its edge. As such, its intensity rises more brightly overhead and gradually declines towards its edge.
Humidity
As sunlight passes through our atmosphere, its light is absorbed and dispersed by gas molecules in various ways, with shorter wavelengths like blue and violet being more easily absorbed than longer ones such as red, orange, and yellow – resulting in the sky appearing bluer when seen from Earth than when cloudy or rainy days exist. This phenomenon also accounts for why clear days appear brighter.
At first glance it may seem strange that we see a white Sun with blue sky; however, there’s actually an easy explanation for this phenomenon. When sunlight enters our atmosphere it travels towards ground where air density is denser, where light hits solid surfaces such as rocks. When light hits these hard surfaces it scatters in all directions but tends to bounce off particles nearer to the horizon more readily, leading to blue skies at first and slowly changing into white once closer to the Sun.
Humidity and particulate matter in the air also influence the hue of the sky, with humid conditions often having more dust and pollution in comparison with dry conditions, often making humid skies appear less blue than others – an effect which can be observed when comparing photographs taken by astronauts flying through space with photos from Earth taken by Earthlings on land.
Sunlight reaching Mars has more of a salmon hue than what we are used to seeing here on Earth due to a persistent dust cloud lingering above it which alters how light is absorbed and scattered by its atmosphere.
Although less direct exposure occurs with blue light from the Sun, there still exists a significant amount of blue reaching your eye from its source. Unfortunately, its wavelength is more strongly filtered by atmospheric gases, meaning you don’t experience as much direct contact. This is why sunrays become more apparent at sunset and sunrise when they must travel through 30x more atmosphere than at midday; their travel also needs to take them through Titan’s dusty atmosphere which gives its butterscotch hue.