Answering this question is more complex than simply saying it looks blue. Sunlight passes through the atmosphere and is dispersed into all directions by air molecules; shorter wavelengths like blue light tend to be dispersed more strongly by these air molecules than longer ones such as UV.
As such, this is why the sky appears blue in color. At sunset when more of the Sun’s light has to pass through more atmosphere layers and scatter into an array of blue-violet wavelengths before ultimately leaving orange-red ones behind.
Light Wavelengths
Light travels through the air in waves. Each wave has a specific length (or wavelength), which we can measure. Shorter wavelengths like blue and violet tend to get scattered by air molecules more readily while longer ones like red and orange tend to pass straight through into our eyes.
The color of the sky depends on how effectively different wavelengths of light are scattered; blue light scattering is most efficient while red light has poor scattering efficiency; therefore during daytime hours the sky often appears bluer.
Scientists refer to this process as Rayleigh scattering. Our atmosphere primarily comprises oxygen and nitrogen molecules which are smaller than visible light’s wavelengths; when light hits these particles it scatters in all directions but especially short blue wavelengths due to having shorter wavelengths than other colors of sunlight.
As a result, blue light can be more intense for our eyes than other hues, creating an overpowering blue tone while all other colors converge into white hues. Blue and violet wavelengths have similar impacts but at much reduced intensities.
As the sun is setting or rising, its sunlight has to pass through more atmosphere before it reaches us, which gives it more time to be scattered and dispersed into various hues – reddish orange at sunset and yellow at sunrise being two examples of these effects.
The color of the sky varies based on what else is in the atmosphere, including clouds, dust, pollution, and water vapor. Particles larger than wavelengths of light scatter all colors equally; smaller particles might only scatter blue light.
As you travel around Earth, you will observe that at noon when the sun is directly overhead, the sky appears blue; but as the sun sets and rises again, its light has had more time to scatter into various colors, with red becoming the dominant one.
Atmosphere Scattering
As sunlight penetrates Earth’s atmosphere it is scattered by particles of various sizes, but blue light scatters more than other colors – giving the sky its iconic blue hue. The effect is strongest near the horizon but can occur throughout. At the horizon the air path is 10X longer so more time exists to scatter blue light multiple times before reaching your eyes – with less intensity nearer its surface of Earth but still noticeable effects.
Earth’s atmosphere consists of molecules much smaller than the wavelengths of visible light, making it easy for them to scatter or absorb any color of visible light that passes through it. Oxygen and nitrogen molecules in particular excel at scattering blue light – known as Rayleigh scattering, this phenomenon accounts for why our skies appear blue rather than violet.
Atmospheric conditions also have an effect on Rayleigh scattering. For instance, clouds and dust haze scatter light less efficiently than clear skies. Haze may consist of tiny droplets of water or particles of dirt and dust – whether formed naturally such as forest fires or volcanic eruptions or artificially such as pollution and traffic emissions.
Earth’s atmosphere reflects some of the sun’s light back down onto itself, creating a bright or yellow sky depending on where this reflection takes place.
Unexpectedly, the setting Sun appears reddish orange because its light takes longer to reach your eyes when it is low on the horizon due to having to travel through more of the atmosphere than when directly overhead.
Ozone in the upper atmosphere has the capacity to reduce Rayleigh scattering, thus rendering the sky appear less blue. Ozone absorbs blue and violet light while letting through more red and green wavelengths; this effect becomes particularly evident at solar zenith angles (SZAs) of 90 and above. To calculate these effects of ozone exposure on sky blueness, authors used SCIATRAN radiative transfer model and CIE chromaticity diagram respectively.
Blue Light
As sunlight travels through the atmosphere it gets scattered by air molecules. Since blue light has shorter wavelengths than red, green, or violet lights it gets scattered more strongly; eventually however, all these scattered wavelengths combine together into white light which our eyes perceive.
Sunlight that passes through the atmosphere becomes polarized as it interacts with air molecules, and when this light hits our retina at the back of our eyeballs it creates a sensation called glare that we become acutely aware of when outside or peering through windows. Polarization makes things appear brighter but may also alter their color cast and cause eyestrain and headaches.
Our eyes are capable of filtering out glare by absorbing UV rays, but they cannot block out all blue light that reaches their retinas. Therefore, sunglasses must be worn to shield eyes from too much direct sunlight.
Sky appears brightest when directly overhead and gradually fades to pale blue near the horizon, as light has to travel through more atmospheric molecules closer to Earth – meaning more blue light gets scattered out and reds and greens tend to be absorbed more readily than their counterparts.
At sunset and sunrise, the sun’s path through the atmosphere takes longer, meaning more blue light is scattered before reaching us and we see more orange and red wavelengths instead. Rayleigh scattering occurs as well – although not as strongly. Indigo and violet wavelengths don’t scatter as efficiently due to less powerful Rayleigh effect than blue. Yet blue light still has enough intensity to stimulate blue cones in our eyes!
Red Light
Visible light spectrum comprises all the colors of the rainbow from violet to red. Each hue has its own wavelength, frequency, and energy values with blue having shorter wavelengths than red. When sunlight enters our atmosphere it may be scattered by air particles into different directions by small atoms or molecules that scatter it unequally; violet and blue light tends to get scattered more than other wavelengths, hence why the sky looks blue during daylight hours.
Daylight sunlight travels more slowly through the atmosphere before reaching our eyes, meaning blue wavelengths scatter more easily and are scattered more widely, giving the sky its signature blue hue.
At night, the sun is lower in the sky and has less time to travel through the atmosphere before setting. This makes the sky appear redder than during daytime; more light is being directed away from its source due to atmospheric particles and gases scattering it unevenly due to being smaller than wavelengths of red light and thus redirecting some light redirected away from its source by their composition.
As you climb higher in altitude, the sky appears paler as atmospheric particles decrease; this means that scattered blue wavelengths from sunlight must travel further through the atmosphere to reach our eyes and become weaker as their travel distance increases.
At sunrise or sunset in clearer skies, you might spot pink and orange streaks of sunlight surrounding the sun at sunrise and sunset, which are caused by dust particles ejected into the atmosphere by forest fires or volcanic eruptions. Cities that experience more pollution also often exhibit orange-red sunsets due to aerosols produced by humans released into the air.