At noontime, the sky appears blue due to molecules in the atmosphere scattering blue light more efficiently than red. At sunrise and sunset, however, its color changes to red due to light having made its journey through an extended path through our atmosphere.
There are various explanations as to why the sky is blue; some such as Rayleigh Scattering are apparent, while others require deeper knowledge of particle physics.
Rayleigh Scattering
As sunlight enters our atmosphere, it passes through a mixture of gases including nitrogen and oxygen as well as trace amounts of other elements like carbon dioxide and methane. When sunlight hits this mixture of gases, its light becomes scattered by their molecules which re-direct it in different directions to change its hue; blue/violet wavelength light scattering is strongest which explains why our skies appear blue during the daytime hours.
Rayleigh scattering was named for John William Strutt, 3rd Baron Rayleigh who first explained it in 1881. There have been various misconceptions and inconsistencies concerning its functioning, yet this article offers the most accurate, widely accepted explanation for its functioning.
Sunlight contains all of the colors of the rainbow, yet humans are most sensitive to wavelengths in the blue and violet parts of the spectrum. As light travels through atmospheric molecules, its path is mostly directed in this region – meaning blues and violets from sunlight tend to escape our line of sight during daylight hours and appear as white washes instead.
Polarization causes the sky to appear slightly bluer at night due to a lower Sun zenith angle at this time, meaning closer proximity and dense atmospheric conditions that allow shorter wavelengths to scatter more effectively and thus creates stronger polarization, darkening the nighttime sky further.
There are a number of other factors which contribute to the daytime blue hue of the sky, including ozone absorption and our eyes being particularly sensitive to blue light. Furthermore, an increase in solar zenith angle may influence why the sky appears blue during daylight hours but orange at dawn and dusk.
Due to these factors, light entering your eyes during the day must travel further through the atmosphere in order to reach retinas, meaning shorter wavelengths like blue are likely to scatter off-path while longer ones like red and orange can pass unimpeded through our line of sight.
Ozone
Media coverage often refers to Earth’s ozone layer as an “ozone hole,” yet its existence remains very important to our planet and sky. Ozone gas, with a pale blue-violet hue, absorbs some ultraviolet radiation that would otherwise reach us and harm our skin; without it, UV levels would increase significantly and the sky would appear yellow or orange instead of blue.
Light entering the atmosphere strikes gas molecules and becomes scattered in all directions, with shorter wavelengths (blue) dispersed more widely than longer ones (red), so sunlight that reaches your eyes appears bluer.
Rayleigh scattering alone cannot account for all the variations in sky color; additional influences include ozone’s influence. Ozone lies close to Earth, so its close proximity absorbs much of its UV radiation from reaching our eyes, giving sky its characteristic blue hue.
One reason the sky appears blue when clear is due to low ozone densities that allow light through easily; this gives rise to its blue appearance when visible sunlight hits it directly.
An interesting phenomenon to keep an eye out for at sunset and sunrise when the sun is nearing the horizon is that its light must travel through more parts of the atmosphere, scattering more blue light which then gets absorbed by larger particles like dust, water droplets, or ice crystals – creating a paler and whiter sky than when directly overhead.
These are only some of the many factors contributing to why our skies are blue; without all the scattering and absorption taking place, it would likely be black. Luckily, however, natural processes protect us from too much UV radiation, keeping skies beautiful yet safe to enjoy.
Clouds
As sunlight passes through Earth’s atmosphere, it comes into contact with gas molecules and small particles, interacting with them to cause its light to be scattered; shorter blue wavelengths being scattered more strongly than longer red wavelengths; this phenomenon is known as Rayleigh scattering after its discovery by an English scientist in 1881, and is one of the primary factors in why skies appear blue.
The color of the sky varies with weather and time of day; during the day it can appear gray; at sunset and sunrise it turns bluish as more light from the Sun has had to travel through atmosphere, scattering away shorter blue wavelengths more readily than longer red and orange ones.
Clouds add another element to the sky’s blue hue. They form when water vapor condenses into tiny droplets of liquid or ice in the air when temperatures are low enough for molecules in the atmosphere to become saturated with water vapor, with millions of tiny salt, dust and smoke particles acting as condensation nuclei to assist this process. As soon as this occurs, air becomes turbulent, leading to all manner of clouds to form, from large puffs billowing toward the Sun to thin wisps slinking past the Moon.
Cloud colors depend heavily on weather conditions; cumulus clouds tend to be fluffy and white when winds are still, while feathery-thin cirrus are seen when winds increase significantly. Yellowish or brownish tinted clouds may indicate air pollution.
Salt or dropping ash into the atmosphere can alter cloud colors by altering how clouds absorb and scatter sunlight, altering its hue as it passes through them and changing how we view it from various locations on earth or space. When seen from different perspectives on either ground or space, we see different hues.
The Sun
When looking up at the sky during the day, it appears blue. While you might think this color reflects ocean waves or is due to oxygen’s blue-tinged gas properties, there’s actually another scientific explanation. Air molecules scatter different-length wavelengths of light that cause this colour change.
The Sun emits an array of colors visible to our senses, but as its light travels through the atmosphere and collides with air molecules, its light becomes dispersed and scattered – longer wavelengths such as reds and oranges are more likely to be absorbed than shorter ones like blues and violets – an effect known as Rayleigh Scattering which gives the sky its blue hue.
At day, the sky appears blue; at night, however, its color changes drastically. Less gas molecules exist at night so less light scatters; additionally, as solar temperatures decrease at nighttime it takes longer for sunlight to reach Earth’s surface and this means more of the blue wavelengths are scattered away before reaching your eyes, leading to its red hue at nighttime.
There are other factors that contribute to making the sky appear different colors, such as clouds and ozone; however, Rayleigh Scattering remains more significant as Sun rays pass through atmosphere at lower angles than during daytime. This causes more of the shorter wavelengths to be scattered before reaching your eyes, giving the sky its distinctive blue hue. A similar process happens on other planets with their own atmospheres; however, results can differ slightly; for instance Mars has a thinner atmosphere than Earth, so its skies appear more white than blue; while Venus’ thicker and reddish atmosphere causes its sky to appear orange than blue.