As sunlight passes through Earth’s atmosphere, its light becomes scattered by gases and particles; blue and violet wavelengths tend to get scattered more than red ones.
Sky has its signature blue color due to our eyes’ greater sensitivity to blue wavelengths, while our bodies excel at absorbing reds and oranges.
Blue Light
Sunlight that reaches Earth is scattered into all directions by tiny air molecules in our planet’s atmosphere, as the light passes through and causes these molecules to oscillate at different rates depending on its wavelength; shorter wavelengths, such as blue light, are scattered more quickly than longer ones like red. This phenomenon accounts for why our skies appear blue during the daytime.
Violet has shorter wavelengths than blue and thus tends to be scattered less. Our eyes tend to be more sensitive to blue; thus it is the color we are more likely to encounter most often.
When sunlight reaches the ocean, its water is colored blue because its blue wavelengths reflect more effectively and absorb fewer red ones; this phenomenon also accounts for why the sky appears blue at sunset but red at sunrise.
As most atmospheric gases absorb radiation in the infrared and ultraviolet spectrums (with some notable exceptions such as ozone), most radiation that hits Earth’s surface is scattered away by these molecules, leaving only blue wavelengths reaching our eyes.
Clear skies appear lighter due to fewer large particles strewing through the air that scatter long wavelengths, while when cloudy it may appear whitish due to larger particles scattering blue and violet light but not red or indigo wavelengths.
Why does the sky look blue on a clear day? During the day, air molecules tend to oscillate faster when exposed to electromagnetic waves with frequencies in the blue part of the spectrum, leading to stronger scattering of sunlight over this part of its spectrum and thus making the sky appear bluer.
Rayleigh Scattering
As sunlight passes through the atmosphere, it encounters oxygen and nitrogen molecules smaller than light’s wavelengths which then scatter it; shorter wavelengths (blue and violet) being scattered more than longer wavelengths (red and orange) thus making the sky appear bluer.
When sunlight strikes clouds, it interacts with water droplets which are much larger than atmospheric molecules and cannot scatter blue light as effectively; as a result, the sky appears paler blue due to more light being scattered by large water droplets than by atmospheric molecules.
Light passing through the atmosphere is further warped by refraction, becoming bent as it travels through it and bends at different angles; red and blue hues in particular tend to become disfigured due to this distortion, explaining why sunset occurs red while sunrise occurs blue.
As the sun ascends and sets, its light passes through more of the atmosphere than at other times of day, meaning more red and orange wavelengths make their way through to your eye than blue wavelengths – giving rise to reddish-orange skies at dawn and dusk and still blue hues at noon.
Mie scattering adds another form of light scattering that contributes to its blue hue: Mie scattering is caused when higher frequency light waves scatter more easily through molecules in the sky’s molecules, making blue light particularly susceptible to this form of scattering; other colors, however, tend not to be affected as greatly and therefore pass more freely through its atmosphere.
Horizontal Scattering
Light travels through the atmosphere where it encounters gas molecules which scatter it, dispersing blue wavelengths more than other colors to give the sky its characteristic color palette. When the sun is low in the sky more of its sunlight must travel through this process to reach you; more scattering means less blue light reaches us while more red/orange hues reach you and creates what we know as sunset or sunrise.
Air molecules are more efficient at scattering short wavelengths than long ones, which is why the sky looks blue during the day but turns red/orange at sunset and sunrise. On the other hand, when water droplets or ice crystals form clouds in the sky they scatter all wavelengths evenly to give clouds their white appearance.
Sunlight that penetrates Earth’s atmosphere reflects off of its surface and gets scattered by gases in its atmosphere, such as nitrogen and oxygen molecules or dust or smoke particles, with smaller particles scattering more light than larger ones. While nitrogen and oxygen molecules do a good job at dispersing shorter wavelengths (violet, blue and green) of visible light (which causes blue skies during daytime), they don’t do as well at dispersing longer wavelengths (yellow orange red) which means skies tend to remain blue during daytime.
Atmospheric composition can vary significantly between planets. Mars features a very thin atmosphere composed primarily of carbon dioxide and fine dust particles; Venus contains pollutants such as smog that make its skies appear red/orange at sunset and purple during a lunar eclipse.
Scattering of sunlight by the atmosphere can interfere with instruments’ attempts to measure horizontal wind at any given altitude. If atmospheric refraction alters light speed, measurements could become inaccurate, which may cause doppler shifts and errors in measured wind. When atmospheric refraction remains constant, errors cancel each other out and provide accurate readings.
Elevation Scattering
As sunlight enters Earth’s atmosphere it is dispersed into all directions by various gases and particles; this phenomenon is known as Rayleigh Scattering and accounts for why skies appear blue in color.
As soon as light reaches Earth’s surface, its scattering mixes up its colors into what we perceive to be white light spectrum. But as soon as it passes back through atmosphere again, its colors become scattered again; at higher elevations where molecules in air are further apart, light scatters more evenly causing sky to look bluer over time.
Violet and indigo light wavelengths have shorter wavelengths than blue light, causing it to scatter less frequently, making our eyes less sensitive to them, making them less prominent, while blue light scatters multiple times, breaking its polarization more easily for us to perceive.
Light scatters through airborne dust particles at lower altitudes and is scattered more widely by these smaller particles, leading to more unpolarized light reflecting off them and making the sky appear bluer at these lower altitudes. This phenomenon accounts for why lower altitude skies appear bluer.
Mie Scattering can also occur in the atmosphere. When light waves hit clouds, they are dispersed into all directions but preferentially scattered by their particles inside; this results in seeing both blue and white light when looking up into the skies.
At sunset or sunrise, only red low-frequency light penetrates through the atmosphere to reach our eyes due to blue frequencies being dispersed more by gas molecules than by larger particles within clouds.
Looking through a telescope allows us to see visible light more naturally and less strongly polarized due to a material used for its polarizer that is more neutral than gases and particles in our environment.