Why the sky is blue is one of the great perennial questions posed by children aged five or under, never ceasing to fascinate.
Sunlight is composed of all wavelengths evenly, but on its path towards Earth’s atmosphere it becomes scattered, with blue wavelengths being dispersed more readily than other ones.
Sunlight
On a sunny day, your eyes perceive the atmosphere as blue. Why is that? Clearly not due to air composed mainly of nitrogen and oxygen gasses that don’t have any color of their own; rather it is caused by sunlight reflecting off molecules in the atmosphere and scattering off of atoms, leading to light that strongly reflects toward your eyes – giving the sky its color. Shorter wavelengths like blue are scattered more strongly due to being closer in frequency range to be detected by human eyes than longer ones such as red.
Rayleigh scattering occurs when sunlight hits molecules in the atmosphere and they vibrate and scatter it, leading to vibratory motion of molecules and reflection in a manner known as Rayleigh scattering – named for the physicist who first identified it in 1858. As part of Rayleigh scattering, most light reflected back out appears blue due to dense atoms and molecules which make blue light bounce around more than other colors.
if it weren’t for Earth’s atmosphere, the sky would appear pitch black, as in outer space. But this thin layer of gases encasing our planet contains molecules and atoms which scatter light from the Sun into blue hues to make its sky seem more vibrant.
Keep this in mind when thinking about white light: all its different hues combine to form it. By shining a flashlight through a prism, you can observe all these different hues that combine to form what we perceive as blue sky.
So if your five-year-old asks “Why is the sky blue?” you can explain that sunlight is being scattered by all those particles in the air and that short wavelengths like blue get scattered more than long wavelengths like red do. Additionally, this also explains why sunrises and sunsets appear orange or red due to light passing through more and more atmosphere; light loses its blueness over time as its path through.
Atmosphere
Our planet’s atmosphere plays an integral role in shaping its hue. When sunlight passes through our atmosphere, it is refracted and scattered by airborne dust particles, creating blue hues due to more blue light being scattered than other colors due to having shorter wavelengths.
Rayleigh Scattering was named for Lord Rayleigh who first defined its equation in 1871. Rayleigh’s equation indicates that the intensity of light that hits an Earth medium, such as atmosphere or surface water, is proportional to its fourth power wavelength; consequently, short wavelengths like blue and violet light tend to scatter more than longer ones such as red or orange wavelengths.
Ozone gas in our atmosphere absorbs certain frequencies of sunlight to create the blue-tinged hue we see today; two-thirds of its blue color come from it as well as its part in creating our planet’s hazy atmosphere.
However, when ozone levels reach sufficient levels, the sky can appear whiter. This occurs because less UV radiation from the sun is absorbed by ozone molecules than usual and more wavelengths reach our eyes more readily. Furthermore, ozone doesn’t always remain present due to nitrogen and oxygen molecules present which may replace it in the atmosphere.
Clouds
Light waves reaching Earth’s atmosphere become scattered by various gases and particles present, resulting in different colors being dispersed at different rates; blue lightwaves dissipate at an increased rate than any other color, leading to our perception of blue skies.
Clouds can alter the color of the sky in unexpected ways. At sunrise or sunset, clouds often take on a yellow or red hue due to light passing through them being dispersed more by its red and yellow wavelengths than its blue wavelengths; making the actual white clouds appear yellow or red in hue.
But that isn’t always the case; clouds may sometimes appear white due to different types of scattering. When water droplets in clouds reach a certain size, they can experience Mie scattering which occurs when their size matches that of light waves; this allows Mie scattering to occur more evenly while spreading out light as da Vinci described (See: Leonardo Da Vinci’s explanation for why sky is blue).
As mentioned previously, the fact that the sun emits more blue light than violet has no bearing on why our skies appear blue; rather it depends on how its light is dispersed through molecules in our atmosphere and water droplets in clouds.
Another significant reason the sky is blue is because our eyes are most sensitive to blue light, while less so to violet and indigo hues. When we gaze up into a sunny sky on a bright day, our brains interpret its color as blue.
Dust
The blue hue of the sky can be explained by light scattering in Earth’s atmosphere. As light passes through air, it is scattered by gases and dust particles present, with shorter wavelengths of blue light being scattered more than longer wavelengths of red light; creating the distinctively blue appearance that characterizes our skies today. Furthermore, ozone gas absorbs ultraviolet radiation contributing to this hue as well.
John Tyndall first discovered this effect in 1859. He observed that when shining white light through clear liquid containing dust or other small particles suspended in suspension, its reflection will appear reddish when seen from the side and blue when observed directly due to longer wavelengths of red light being absorbed by particles while shorter blue wavelengths scattered.
However, this explanation is far more complex. Light from the sun does not enter our atmosphere at a uniform rate and as it travels higher up, its density decreases. This causes wavelengths of sunlight to bend more strongly as they pass through, thus decreasing how much blue light reaches our eyes; most of it coming from above us while what reaches below is paler or whiter in comparison.
Other factors can also play a part in altering the hue of the sky, including cloud cover and particle types found in the atmosphere. Pollution or smoke particles in particular can turn skies grayer while volcanic ash deposits make the sky appear redder or orange.
Though many factors contribute to its color, one key reason the sky appears blue is due to scattering between shorter wavelengths of blue light and longer wavelengths of red light. This scattering effect increases with lower densities of the atmosphere and viewing angles; Edward Hulburt conducted research in 1953 that demonstrated Rayleigh scattering accounted for two-thirds of its blue hue; further contributing is ozone gas at sunset zenith angles adding extra vibrance.