Oliver Scott of Wombarra Our atmosphere interacts with sunlight to produce blue hued skies on Earth. Blue light scatters more readily than other colours; thus creating its distinct blue color.
Sunlight contains all of the colors of the rainbow, which we can see when passing it through a prism. But when the sun is low on the horizon at sunset or sunrise, its light has to travel farther through our atmosphere before reaching you and thus becomes even more blue-scattered before reaching you.
Light Scattering
Light traveling through the atmosphere scatters in all directions, producing a pale and blue glow as its path passes over particles in the atmosphere (including air molecules). This phenomenon, which was first identified by 19th-century British physicist Lord Rayleigh, is known as Rayleigh scattering.
On a clear day, when looking up at the sky you will likely witness scattered blue light that is lighter than direct sunlight coming directly from the Sun due to more atmospheric molecules blocking its path through lower elevations than at higher altitudes.
As you travel higher into the atmosphere, blue light becomes even paler as more atmosphere has to pass through before reaching your eyes. Additionally, many scattered beams reach only some parts of their original path before reaching all your senses.
Light of all wavelengths scatters in the atmosphere, but air molecules tend to scatter shorter-wavelength light such as blue and violet more readily due to having closer wavelengths with vibration frequencies of electrons in molecules – when these electrons vibrate they radiate energy and give the sky its signature blue hue.
Red light wavelengths are longer, making them less vulnerable to scattering; this explains why astronauts see Earth as red rather than blue from space, and why sunrise and sunset make more visible red hues than usual.
Refraction and total internal reflection cause multicolored light to bend into various wavelengths, while scattering leads to its blue hue. It varies based on how much of each type of particle there are in the atmosphere – this depends on where you are.
Atmosphere
As sunlight travels through our atmosphere, it becomes scattered by gas molecules. Blue light tends to be scattered more than red or orange light due to its shorter wavelength; when struck by air molecules that are smaller than itself, these vibrate, producing Rayleigh scattering – named for the scientist who first discovered this form of electromagnetic radiation.
When blue light strikes water molecules in our oceans and lakes, it takes on a deeper blue hue due to the water molecules’ ability to absorb certain wavelengths of light – not because of what lies above but because water molecules excel at absorbing short blue and violet visible light waves while not being as effective at absorbing longer red or orange visible light waves; hence why open sea appears dark blue.
As sunlight travels through our atmosphere, its hue may shift toward red near the horizon due to blue light having to travel further through it before reaching you and being scattered away in other directions. As one ascends higher into our atmosphere, less blue light gets scattered; thus giving our skies above a distinctive dark or bluish violet appearance.
As you ascend into the stratosphere, your atmosphere thins further as its temperature increases with altitude. This layer extends approximately 30 miles above Earth’s surface and features dry air with few clouds, perfect for airplane flights as it rarely experiences turbulent airflow. Airplanes generally fly within this layer without encountering much turbulence as a result.
At even higher altitudes lies the exosphere – our atmosphere’s outermost layer, composed of extremely thin and sparse molecules. The exosphere contains most of our planet’s oxygen supply as well as creating a clear starry night sky due to sodium’s presence here. You might even spot shooting stars with distinct hues!
Sunlight
Our eyes are most sensitive to blue light, so when combined with all of its other hues we perceive the sky as blue. Otherwise, any remaining sunlight may become mixed with different colors to appear white – but if our sun were instead emitting violet or red wavelengths instead, our view may change drastically and reveal more purple or red shades instead.
At nighttime, the sky becomes blue due to sunlight scattering through the atmosphere. At higher elevations of atmosphere, more air molecules come into contact with it, increasing its likelihood of scattering shorter wavelengths such as blue light. When this happens, longer wavelengths remain that cause it to mix together to appear white in appearance.
If you were standing on another planet like Mars, the sky might appear different than on Earth. Because the atmosphere on Mars is much thinner and less likely to scatter blue light, its sky typically displays as darker shades of blue or even bluish violet hues.
Ocean and lake waters appear blue due to how light interacts with them: long wavelengths of red and orange light absorb into the sea or lake water, leaving only blue light visible for you to see.
At sunrise and sunset, the Sun appears lower in the sky than during daytime. This requires it to travel through a greater portion of atmosphere before reaching your eyes – more blue light is scattered by this process, while shorter wavelengths (red and violet) pass straight through without scattering into space. As a result, sunrise and sunset seem to alter its hue as they pass from horizon to horizon.
Sunset
Sunsets are one of the most spectacular natural events you can witness, offering an ideal opportunity to step back from daily stressors and experience nature’s beauty. Watching an idyllic sunset is great for mind, body, and soul; not to mention great for eyesight! But have you ever wondered why the sky becomes blue at sunset? That is because our atmosphere scatters sunlight differently during these evening moments.
As the sun lowers toward the horizon, its light must travel through more of our atmosphere than when it is higher up in the sky. More atmosphere means more air molecules which scatter violet and blue wavelengths away from your line of sight while red and orange wavelengths have difficulty being dispersed – this explains why sunset skies tend to feature more vibrant red-orange hues than blue hues.
At sunrise and sunset, the sky appears more yellow due to the Sun passing closer to Earth’s surface – meaning a thinner atmosphere layer which scatters shorter wavelengths more readily – while red and orange wavelengths have more of a chance of making their way through.
As you ascend in altitude, the amount of blue light that penetrates the atmosphere decreases due to fewer gas molecules being available to scatter it; this causes skies at high elevations to appear paler or even whiter due to less of its blue wavelengths reaching your eyes.
Another frequently-asked question about the sky concerns why it darkens at night. One reason could be due to how the Moon lacks an atmosphere; therefore there are no gases to diffuse its light, leading to almost 100% direct illumination with greater luminosity than would exist here on Earth.