As sunlight passes through the atmosphere it is scattered by air molecules and water droplets into scattered light of shorter wavelengths (blue). This causes it to appear blue during the day.
Clouds form when larger particles such as water droplets spread all wavelengths of light equally, creating white clouds. If there is too much dust or haze present, however, then their appearance will change into gray ones.
The Sun
The blue sky is one of the world’s most beloved images. People everywhere expect to see it when looking up into the skies – from cloudy Seattle to polluted Mexico City – and it features prominently in every painting, photograph and postcard we own.
Answer lies within our atmosphere’s composition. When sunlight hits our atmosphere it gets scattered by gases and particles found therein – particularly smaller molecules than visible light wavelengths, which tend to scatter blue light more than any other colour – due to which eyes being more sensitive to this wavelength makes sky generally bluer than other hues.
Clouds appear white due to water droplets contained within them that scatter all colors evenly – this explains why you usually see blue skies without clouds, while their presence appears as white when present.
On rare occasions when you observe non-blue skies, they are most often due to atmospheric haze or dust particles obstructing sunlight’s path through thicker atmosphere layers – this causes its shorter wavelengths to be stripped from sunlight’s path, turning its color redder than usual – much like how sunrise and sunset appear reddish in hue.
There are various causes behind why the skies around Earth take on different colours. One simple factor is water vapor in the atmosphere absorbing certain frequencies of light and refracting them differently (known as Rayleigh scattering), thus altering how much blue and other hues of light scatter; sometimes leading to rainbow formation in the sky.
Other variations can be seen due to chemically rich air or uneven cloud surfaces; however, on the Moon it’s impossible for any other colours other than black to be seen due to the absence of atmosphere. If this were present on Mars it likely would produce a butterscotch or burnt orange hue as particles absorb and scatter sunlight at different rates.
The Atmosphere
A clear blue sky appears that way because atoms and molecules in Earth’s atmosphere scatter light waves that reach it – this phenomenon is called Rayleigh scattering, and is responsible for giving it its distinctive hue.
Violet, indigo and blue wavelengths scatter more easily due to atoms and molecules in the atmosphere being much smaller than sunlight’s wavelengths.
When sunlight hits atoms and molecules, its light scatters all directions with more intensity for shorter wavelengths, including blue wavelengths. When this happens, it creates the appearance of blue skies.
Clouds appear white because they consist of water droplets larger than atmospheric molecules and therefore disperse all wavelengths of sunlight evenly, giving a white appearance from Earth. Furthermore, there may also be trace amounts of blue pigment added into the atmosphere which adds depth and dimension to its overall blue appearance.
Scientists Tyndall and Rayleigh initially hypothesized that the blue hue of the sky could be explained by small particles of dust or droplets of water vapour in the atmosphere, however this theory has long since been disproved; Mie scattering, which is a type of refraction is the true explanation behind its color.
Light waves hit surfaces that have different refractive index values and can then be redirected in various directions by hitting an object such as water droplets with higher refractive index than surrounding air, for instance. Light may then appear closer than it actually is to viewers viewing them through such lenses.
This effect may also cause the sky to appear greenish, such as when sunrise or sunset takes place when the sun passes through a greater proportion of atmospheric gases on its journey down towards Earth – this increases blue wavelength scattering while simultaneously allowing more red and yellow wavelengths through and being seen by us.
Water Droplets
Rayleigh scattering occurs when sunlight strikes our atmosphere and is reflected off gasses in it, giving rise to reflection off atmospheric gasses known as Rayleigh Scattering; named for physicist Lord Rayleigh. Rayleigh Scattering occurs because atoms and molecules that make up gases in our atmosphere are smaller than wavelengths of light; short waves (violet/indigo) get scattered more than longer waves (yellow/orange/red); this gives the sky its blue hue.
As sunlight travels through clouds, it interacts with large water droplets which interact with light waves emitted by sunrays, scattering all colors equally across their wavelengths to give white hues to clouds.
Polarizing sunlight means reflecting different wavelengths in different directions, creating an effect whereby darker hues appear when seen through a polarizer.
As sunlight passes through clouds, its energy is dispersed either upward or laterally to its sides, giving rise to pearlescent colors in the sky and giving a whiter appearance on either the tops and sides than its base which receives less direct light. This explains why pearlescent hues appear.
Sometimes the sky can take on an orange or red tint due to sunlight passing through more atmosphere than normal. This causes more of the blues to be scattered away while more red and yellow colors reach your eyes; hence why sunsets can often appear so vibrant.
As there is no atmosphere on the Moon, no Rayleigh scattering takes place and instead the surface looks creamy white due to abundant iron found on its dusty surface – hence why lunar eclipses appear blood red!
Dust
The sky is blue because tiny particles in the atmosphere scatter short wavelengths more efficiently than long ones, due to Rayleigh Scattering – this process involves air molecules dispersing short wavelengths such as blue light through their molecules more frequently than long ones, which allows only blue light rays to reflect from atmospheric layers and clouds.
As a result, the sky tends to be bright blue during the day and grey at night, though occasionally can vary between blue and grey depending on weather conditions. When there’s lots of dust or pollution present in the air, however, the sky can appear yellow or orange due to particles being smaller than wavelengths of sunlight reaching it.
But even without dust or pollution, the sky can still appear blue for other reasons. One possible scenario involves temperature inversion; warmer air rises while cool air descends, creating an effect similar to that of rising air mass rising and cool air falling resulting in an overall blue hue to the sky.
There are a few simple experiments you can conduct to help explain why the sky is blue and clouds are white. One such experiment entails using a flashlight, some milk/cream and a transparent container. First step should be adding some milk into container before dropping some drops of water on it before shining your flashlight onto the mixture and watching as its color shifts under its illumination. Next time around use different colored milk such as red or purple milk instead!
Another way to illustrate why the sky is blue is to study the shadow of a tree on the ground. When directly overhead, this shadow will appear blue while yellow/orange when lower in the sky, because sunlight scatters more blue light than any other color.
Clouds appear white due to the same mechanism that causes skylight to appear blue: when sunlight passes through clouds and interacts with their water droplets and ice crystals, which are larger than wavelengths of light, thus evenly scattering all wavelengths evenly, giving clouds their signature white appearance. Other particles found within clouds may cause them to take on different hues; examples would include volcanic eruptions or forest fires which release particles smaller than wavelengths that scatter blue light more strongly than other colors resulting in multicolored cloudscapes.