Sunlight penetrates our atmosphere and is absorbed and scattered in various ways; blue light is typically absorbed less, and red light more.
This explains why the sky appears blue and sunsets and sunrises appear redder due to more light passing through more atmosphere and reaching your eyes.
Rayleigh Scattering
As sunlight travels through our planet’s atmosphere, it is scattered in many directions by tiny molecules in the air we breathe. Light of different wavelengths scatter more readily in certain directions than others – shorter wavelengths (like blue) being more susceptible than longer ones like red – creating the iconic hue of our skies.
Imagine we’re looking up at a clear tank of water with small amounts of milk or soap suspended within. When we shine a beam of white light through this tank, its light will scatter in all directions; but due to small particles scattering the light having lower mass than those within the tank they scatter blue and violet waves more than red and orange waves causing further scattering of light rays.
Atmospheres and skies alike have much in common. Blue and violet wavelengths of sunlight are scattered by air molecules in our atmosphere – mostly nitrogen, oxygen and various other gases as well as dust particles (primarily dust pollution) – that contribute to creating blue skies. When these air molecules scatter light at their greatest intensity, the sky appears blue.
At sunrise and sunset, when the sun nears its horizon, its path through our atmosphere changes considerably. Since it must travel further and faster through it, its blue wavelengths scatter away more quickly from our eyes to be absorbed by gases in our atmosphere resulting in the vibrant oranges and reds we experience at these times of day.
When light waves are scattered in one direction, their intensity decreases; this is known as spherical aberration and results in their colors looking faded or washed-out. If colors remain relatively unaffected by this type of scattering process, we refer to that color as being nearly polarized – this phenomenon explains why our sky appears blue, since polarization from scattered blue light cancels out any remaining red and orange lights that remain.
Dust & Aerosols
Atmospheric air contains millions of tiny particles known as aerosols, which interact with sunlight when passing through it in various ways. Some light is absorbed while some gets scattered, depending on the particle’s kind and shape – some particles being small and round while others long and thin; whatever its size, aerosols play an integral part in making the sky blue as they scatter more blue wavelengths than any other colors.
Mineral dust from wind erosion is the most ubiquitous aerosol, often traveling thousands of kilometers downwind and fertilizing plant growth in rain-fed rainforests and causing algal blooms in oceans. Other common aerosol types include sea salt, smoke from forest fires or volcanic eruptions and droplets produced from cloud condensation processes; additionally there are man-made (anthropogenic) aerosols produced in factories or from burning fossil fuels that also contribute.
Light that has been scattered by clouds or other particles appears less blue and more yellow and red due to blue wavelengths being scattered more strongly than indigo and violet wavelengths, while red wavelengths tend to pass through our atmosphere more easily, reaching our eyes more readily.
Mars experiences the same process. While its atmosphere may be thin, it still contains particles. When the Sun rises or sets there, its light travels through far more atmosphere than here and thus more molecules interfere with violet and blue wavelengths, which causes them to be diverted away from our line of sight; by contrast pink, orange, and red wavelengths continue down their direct path to our eyes.
From spacecraft, if we examine Earth closely enough, blue wavelengths appear more scattered than any of the other colors and the planet appears bluer than expected. This phenomenon occurs because Earth has a thicker atmosphere than most planets; eventually however, its atmosphere will thin out and other colors will pass more freely through.
High Elevation Scattering
As sunlight enters our atmosphere, it interacts with particles smaller than lightwave wavelengths such as dust specks or nitrogen and oxygen molecules, leading to Rayleigh scattering; shorter wavelengths like violet are scattered more than longer ones like red; the end result being that blue light waves reach our eyes more readily while other colors less so, giving rise to our perception of blue skies.
As we climb higher in altitude, atmospheric molecules decrease, decreasing the amount of blue light reaching our eyes – this explains why the sky becomes darker or bluish-violet as you ascend further in altitude.
As the Sun gets lower, sunrises and sunsets become brighter due to more molecules being struck by its light passing through more molecules in its path; more blue and green light gets scattered away while pinks and yellows make an impressionant appearance that creates magnificent sunrise and sunset colors.
Color of clouds depends on a combination of Rayleigh and Mie scattering, but more importantly on their size and shape. Small particles give clouds their characteristic white appearance; larger ones may give a grayer or even yellower or orange tint depending on dust/haze content present.
As sunlight passes through a cloud, its reflection in all directions is dispersed across its surface, but its polarization depends on which way the cloud moves. Whenever incoming solar energy enters in an opposite polarization pattern from a cloud, its appearance will be white due to all wavelengths being equally scattered across it. If the polarization of sunlight reflected off a cloud is perpendicular, then its appearance will change to gray as different wavelengths of light will be refracted differently and won’t scatter in an even manner. You can see this effect of various solar energies through images like those shown below which show that clouds with no polarization appear white while those with perfect polarization appear grayer.
The Sun
The Sun is the central figure in our solar system. It provides life on Earth with light and heat that sustains it, and also creates and sustains its vast blue bubble – the heliosphere – around which it orbits; this bubble is dominated by magnetic fields emanating outward in spiral shapes like that of garden sprinkler.
Our blue skies are caused by sunlight interacting with molecules and particles in Earth’s atmosphere, particularly shorter wavelengths of blue light that scatter more effectively than longer ones, giving rise to its signature blue hue. This process is known as Rayleigh scattering, first discovered by Lord Rayleigh himself back in 1871.
Early in their scientific explorations, many scientists believed the blue color of our skies was caused by dust and water droplets in the atmosphere. Astronomer Tyndall for example suggested it could be down to “small particles of oxygen and nitrogen in combination with drops of water vapour”. Unfortunately it took science much later to debunk such ideas as being false.
Astronomers and scientists generally agree that our blue skies are caused by interactions among gases and particles in our atmosphere, particularly ozone molecules which absorb blue light while scattering red and other longer wavelengths; as a result, this causes our atmosphere to reflect blue and violet wavelengths, giving it its characteristic hue.
Earth’s oceans appear blue due to how different wavelengths interact with their surfaces. Longer wavelengths, such as red and orange hues, absorb by water molecules to give an appearance of blueness on its surface.
Mars would look dramatically different. Since the Moon has an even thinner atmosphere than Mars, sunlight does not scatter evenly; therefore it appears dark at night while not becoming brighter during the day.