No one can resist a beautiful day with blue skies! These lovely views result from sunlight passing through Earth’s atmosphere and being scattered by oxygen and nitrogen molecules present therein, particularly those with shorter wavelengths, like blue. As light with shorter wavelengths like blue has more chance to get scattered than other colors due to these interactions, we see bluer skies more frequently on such days.
However, red and orange light waves have enough length to avoid scattering in the atmosphere, creating the sky’s characteristic blue color.
Rayleigh Scattering
As sunlight passes through the atmosphere it interacts with gas molecules and is scattered. This process is most noticeable for wavelengths shorter than visible light; blue ends of the spectrum tend to be heavily scattered during daytime hours.
Rayleigh Scattering, named for Sir John William Strutt, 3rd Baron Rayleigh who first described it in 1870s, can be observed today.
All light is scattered, but some colors tend to scatter more strongly than others. Blue and violet light are particularly susceptible to being scattered off-course; blue-violet wavelengths tend to scatter more strongly than red ones due to molecules being smaller than the wavelengths of visible light and therefore increasing chances of colliding with and bouncing off them, changing frequency and direction, resulting in waves with different frequencies that appear as distinct colors in our vision.
Rayleigh scattering alters the amount of sunlight reaching the horizon, which accounts for its paler color as you approach. As you move higher into the atmosphere, there are fewer molecules to scatter light – only blue wavelengths reach your eyes, giving an overall dark blue appearance to the sky.
Remind yourself that the color of the sky is only an effect of light interacting with our atmosphere; space itself does not possess a fixed hue; tomorrow the Sun could emit different colored light rays than those we see now. Sunrise and sunset can cause the Sun to appear white as its closest point is nearer the horizon; therefore, its color in the sky can be much more complex than that of just plain old sky itself. However, it’s always a treat to witness a clear sky and marvel at its physics. Andrew May is both a science writer and former physicist; earning his Ph.D. in Astrophysics from Manchester University U.K. He has worked in academic, government, and private sectors over 30 years.
Atmosphere
The atmosphere of Earth is a thin, blue-tinged layer of gases and other substances enveloping it like an invisible blanket. Sunlight interacts with this atmosphere to produce colorful rainbows in the sky that can be seen all across its surface.
As soon as sunlight hits our atmosphere, molecules in the air scatter some of it and redirect it in all directions. This scattering process is more powerful for shorter wavelengths (blue and violet) than longer ones; hence we perceive our sky to be bluer.
Violet does not completely dissipate from sunlight, yet. Luckily for us, the sun provides such powerful sources of red and yellow light that any scattered blue light that gets scattered by our atmosphere can still be absorbed by our eyes which are more sensitive to these wavelengths.
Actually, in space or the Moon we would see the sun as white, as there would be no atmosphere to scatter light. This phenomenon explains why open water appears blue: blue-tinted light from our atmosphere gets absorbed by water molecules leaving only red and yellow wavelengths behind.
If the atmosphere were thicker, we’d see nothing but darkness above. That is why it is such a blessing that our atmosphere is thin – giving us beautiful sunrises and sunsets during the day, plus night-time stargazing experiences!
Atmospheric conditions also lend our skin a healthy tan, as the atmosphere scatters red and yellow wavelengths that keep us healthy from reaching our eyes. This phenomenon also accounts for why oceans appear blue – we are most sensitive to wavelengths reflected by water; similarly if you ever witness a total solar eclipse you will notice how less sunlight reaches us; your eyes become most sensitive to wavelengths reflected back from water surface reflection. And should you witness one, your vision will adjust to not receiving enough blue light which keeps you awake; should your body adjust itself in response.
Sunlight
When the sun is out and the sky appears blue, this is due to how light from the Sun passes through Earth’s atmosphere and scatters at shorter wavelengths – mostly blue and violet – leading us to perceive it as such during daylight hours. When sunlight enters our atmosphere it scatters widely but mostly in shorter wavelengths like blue and violet that make blue light stand out more prominently, producing what we perceive as blue skies during daytime hours.
But from space, the sky doesn’t appear blue due to a lack of an atmosphere to scatter sunlight. Without our planet’s atmosphere to scatter it around, sunlight simply travels in an unidirectional path until it meets something which absorbs it or until its energy is absorbed by an object like planet or star surface.
Lakes and oceans appear blue due to reflecting the color of the sky; this occurs because water absorbs red wavelengths while amplifying blue ones, reflecting light back from space without air to reflect it back out again.
At sunset, the sky transforms from blues and violets to pinks and oranges as the Sun passes lower in its orbit, having passed through more atmosphere than at noon. More molecules scatter blue and violet wavelengths while yellow and orange wavelengths pass unharmed through.
If you ever get the opportunity to gaze into space from satellite, planet, or spacecraft, you will notice a stark darkness punctuated with stars and planets. But why doesn’t space seem more colorful? Rayleigh Scattering may hold some answers. As previously discussed, Sun rays contain wavelengths from violet through red; when entering Earth’s atmosphere these shorter blue wavelengths scatter more than longer red ones due to human eyes being more sensitive to blue wavelengths; thus giving rise to our perception of it being blue when high in the sky.
High Elevations
Our atmosphere scatters sunlit light back onto Earth in such a way as to produce blue skies, but this doesn’t apply everywhere on the planet – at higher elevations for example, thinner air allows more of its rays through while less are scattered back and absorbed as shadowy spots on the horizon. Furthermore, depending on factors like pollution or water vapor in the air (Mars has permanent dust clouds that cause its sky to turn butterscotch color), colors of sky can also change based on what’s in our atmosphere – creating different shades from blue skies as seen throughout planet.
On the Moon, due to its lack of an atmosphere, blue skies are impossible. Instead, its surface is blackened by craters and mountains dotting its surface; illumination occurs only at high noon when sunlight directly illuminates it.
At the summit of Mount Everest, the air is so thin it almost feels nonexistent. This isn’t due to fewer molecules; rather it’s because their molecules scatter sunlight differently: blue wavelengths scatter more while violet and red wavelengths absorb more. On clear days this gives the sky its signature blue hue, but on cloudy days blue wavelengths become completely absorbed leaving only white remaining as their reflection on cloud cover obliterates their effect and so appear whiter overall.
At lower elevations, horizons can appear paler because sunlight must travel through more of the atmosphere before reaching you – this means shorter wavelengths are more likely to scatter away, leaving only longer ones that your eyes can detect – leading to grayer or whiter skies nearer the horizon than they do during high noon overhead. Sunset or sunrise often results in redder, purpleer and yellower hues due to more light passing through atmospheric layers than it does at midday.