As sunlight passes through Earth’s atmosphere, molecules of nitrogen and oxygen scatter shorter blue wavelengths more strongly than other colors – an effect known as Rayleigh scattering.
Longer wavelengths like yellow, red and orange pass unimpeded through the atmosphere – so why is the sky light blue? Unfortunately, the answer lies somewhere within.
The Sun’s Rays
As sunlight hits Earth’s atmosphere, it passes through gases and particles that disperse it in all directions, in a process known as Rayleigh scattering (named for Lord Rayleigh who first described it). Light with shorter wavelengths like blue and violet tends to get scattered more than red light; due to our eyes’ sensitivity towards blue hues this usually becomes the dominant hue that reaches us.
At sunrise and sunset, however, the Sun is lower in the sky; therefore its light must travel further through the atmosphere before reaching our eyes. As a result, more blue and green wavelengths are scattered away than usual, leading to more orange and red wavelengths making their way through and giving off vibrant skies at these times.
Rachel White, an atmospheric scientist at the University of British Columbia and an atmospherics professor. Light is said to affect molecules and atoms in the air in various ways; these have various properties such as vibrating quickly or oscillating, in addition to possessing different colors and sizes from small dust particles up to giant gas molecules that make up our skies.
Sunlight can produce an array of colors when it reaches Earth, but most of its visible light spectrum consists of short wavelengths, such as blue and violet hues. When this light reaches our atmosphere it is dispersed through scattering processes similar to those described earlier, with most likely reflecting off gas molecules rather than being absorbed, causing blue skies.
Longer wavelengths corresponding to red and orange light do not scatter as heavily due to more likely absorption by molecules than reflection off other molecules, leading to paler horizons than skylines due to sunlight having traveled further before reaching our eyes.
Indigo and violet wavelengths contribute only slightly to the color of the sky; however, without them it would appear stark. Our eyes respond most favorably to blue wavelengths while being least stimulated by indigo and violet ones.
The Atmosphere
Light travels through the atmosphere, scattering off tiny molecules of nitrogen and oxygen and producing visible blue and violet light as it passes. Light that scatters is what we perceive as blue-violet hues while that which does not scatter continues in its original path; intensity of scattered light depends on frequency with higher frequencies (blue/violet hues) being more strongly scattered than lower frequency (red/yellow hues) light – an effect known as Rayleigh scattering, named for Lord Rayleigh who first proposed its mathematical description in 1871.
As sunlight reaches Earth’s atmosphere it must pass through an enormous volume of air that absorbs and diffuses it; more long wavelength red and yellow light are absorbed and lost before reaching our eyes, while blue and violet wavelengths are scattered much more strongly, giving the sky its signature hue.
Atmospheric conditions also influence the brightness of Sun rays at different times of the day, as can be seen by changes to sky color during sunrise and sunset. At these times, more atmosphere must be traversed before Sunrays emerge and this causes more blue and violet light to scatter away, paling out the sky’s hue in response.
On the Moon, where there is no atmosphere to filter sunlight directly onto your eyes and appear as blue-tinged light; however, planets in our solar system with atmospheres feature skies with various hues; Mars for instance has an orange hue due to the thick layer of carbon dioxide and methane present there.
Photos sent back by Mars rovers have revealed a salmon-colored sky on Mars; although scientists involved with these missions have yet to visit it in person. One possible explanation could be that methane and carbon dioxide in its atmosphere absorb and scatter solar radiation, making the planet appear lighter than expected.
The Horizon
The horizon is the boundary that marks Earth’s surface from its sky. Additionally, this term describes any celestial body with an uneven surface such as planets or stars – whether planets, moons or stars. If floating freely through space (or Earth was transparent), your eyes would see only straight lines between you and ground below – however due to an atmosphere encompassing our planet and thus curving its surface our horizons are usually curvier in form than straight.
Rayleigh scattering explains why the sky appears light blue: when light passes through an atmosphere, its wavelengths get scattered in all directions – more so for shorter blue wavelengths than longer red ones, creating an effect known as Rayleigh dispersion that makes sunlight look more blue than it would if directly from the sun.
As soon as the sun dips below the horizon, such as during sunrise and sunset, its effect becomes even more striking. Light must travel a longer distance through the atmosphere before becoming scattered by thicker layers – leaving more red and orange wavelengths direct which create more vibrant sunset or sunrise scenes.
Rayleigh scattering does not have the sole responsibility for changing the hue of the sky; other factors, including cloud cover, air temperature and humidity levels and weather conditions can alter its hue as well. When there is pollution present in the atmosphere, grey hues tend to dominate; conversely on clear days with optimal meteorological facilities the sky often looks bluer and brighter.
Furthermore, horizons near bodies of water or deserts with steep air temperature gradients often appear darker as warmer air rises above cooler water, creating an illusionary mirage effect whereby it appears higher or further away than it actually is.
High Elevations
As soon as sunlight reaches Earth’s atmosphere, it interacts with gas molecules present. This interaction causes light to be scattered all directions in an effect known as Rayleigh scattering; blue and violet hues tend to scatter more quickly through this process, so they take shorter paths through our atmosphere towards reaching our eyes – thus explaining why the sky appears blue during the daytime hours.
As the sun approaches the horizon during sunset or sunrise, its wavelengths must travel through more atmosphere to reach your eyes – increasing the possibility that longer wavelengths like red and orange light get scattered away, leaving behind only blue light which shines brightly onto your eyeballs – thus turning the sky reddish orange.
What makes the Moon even more intriguing is that, although it doesn’t possess an atmosphere, it still appears dark when seen from Earth. This is likely due to its larger gravitational pull than our planet causing its atmosphere to compress resulting in a thin layer of gas which reflects and scatters visible light rays back towards us.
The Moon also boasts an extremely strong magnetic field which helps deflect some of the Sun’s rays away from hitting Earth directly and making its brightness seem greater than it actually is.
Sunlight travels through Earth’s atmosphere of nitrogen and oxygen atoms on its journey towards your eyes, dispersing light in all directions; blue hues tend to scatter more widely, accounting for why skies appear blue during sunny days.
The sky’s color can change with humidity, dust and pollution levels in the air – cities often feature thick smog that makes skies appear gray or brown while rural areas with less pollution or dust often boast clearer skies. Altitude also plays a part in this; mountains’ air pressure decreases with elevation so when approaching them the skies appear lighter in hue.