Why is the sky blue?

I have so far had the good luck that my triathlon training has taken place under clear blue skies, constantly reminding me of one of the most empowering and transformative lessons I learned in a physics classroom: an answer to the age-old question “why is the sky blue?” To this day I distinctly remember the day after learning this lesson: I was walking to the campus health center with a sinus cold, but soon forgot my sniffles upon stepping outside, looking, up, and experiencing the bluest sky I had ever seen.  I was looking at that sky with a fresh set of eyes – eyes that understood the physical interactions of sunlight with the atmosphere above that gave rise to that pristine blue color.  

We learned in "Galileo and the two cyclists" that even though we sometimes forget that air is there, we can’t ignore its effects.  Today, air is at it once again.  

First, a little bit about the light that gives the sky its color.  Light is a wave, with color determined by its wavelength – the distance between two successive wave crests.  The light shining down on us from our friendly neighborhood star is composed of all the colors of the rainbow (and more that we can’t see: UV that burns or tans our skin, infrared that warms us when we step out from the shade).  The sun is most intense, or brightest, in the visible range of the spectrum (which explains why we have eyes that have evolved to see in the visible spectrum).  When all of these colors combine, we see it as white light (check out this extremely well put-together video on color perception and an awesome podcast by the guys at Radiolab).   

 

The white light from the sun is really a combination of all of the visible colors, and then some.  The graph on the right shows how much light (intensity) of each color (wavelength) the sun produces.

So, we’ve got white light – light of all colors, really – coming in from the sun.  It travels through space unimpeded for a good 8 minutes before hitting the air in our atmosphere.  We tend to think of air as transparent, which is a good approximation when you’re looking at things only as far away as your computer screen.  But if air were completely transparent, light from the sun would just sail past us over our heads, and the daytime sky would be black as night.  You see, light travels in a straight line unless something gets in its way.  You can only see a flashlight beam if it travels through dusty air, for example.  This also explains why all of the photos from the moon landings have a black sky backdrop, even when the sun is high in the sky.  There’s no atmosphere on the moon, which means there are no air particles to bounce the light from the sun down to the surface of the moon.

But here on earth, there are lots of air particles.  Incoming white light bumps into these air particles, causing the light to scatter in every direction: some of it down towards our eyes.  But this should imply that the sky is white – the incoming white light bounces off of the air particles, and is simply redirected down to earth. 

The physics of the light-air interaction is a bit more subtle than that.  The particles in the air – atoms really – can be thought of as an electron and a nucleus bound together by a spring.  Since an electron has negative charge, and the nucleus of an atom – made of protons – has positive charge, they will attract.  The light wave pulls them temporarily apart, but then their electrical attraction brings them back together.  When the white light wave runs into this spring-y atom, it causes the atom to vibrate.  These vibrations re-emit light.  And here’s the key: the re-emitted light is no longer an equal combination of all colors.  The vibrating atom most strongly emits short wavelength light – violet and blue. 

When caused to vibrate by incoming light atoms re-emit, or scatter, blue light much more than red light.

Because the re-emission process favors shorter wavelengths, the light that gets re-directed down to our earth-bound vantage point contains a different mixture of colors than the initial incoming rays – it contains a greater proportion of blue, and much less red and yellow.

Relative to the incoming solar spectrum (dashed line), the scattered light from the atmosphere contains much less yellow, orange, and right light.

This means that the light that gets scattered down to our eyes by the atmosphere no longer looks white (an equal mixture of all colors), but instead takes on that familiar sky blue hue. 

A careful observer will notice that the sky isn’t uniformly blue, however.  The further you look from the sun, the more scattering events had to occur to trace a path from the sun, to that patch of sky, finally to your eye.  Furthest from the sun, the sky is its deepest blue.  Light coming from patches of sky nearer to the sun has taken a more direct route to your eye, has undergone less scattering, and therefore retains its whiteness.

When looking further from the sun, the sky appears more blue because the light had to scatter more to get to your eye.

There is a flipside to this blue sky coin.  We tend to think of the sun as yellow because when it is low enough in the sky that we can comfortably look at it, we are peering through the shroud of the atmosphere.  When looking at the dawn or dusk sun, its light has to travel through a lot of atmosphere – a lot of spring-y atoms – which causes all the blue light to bleed away, leaving mostly reds and yellows – thus the pink, orange, and red skies that we sometimes see in morning and late evening.  And if you live in LA, where there are even more spring-y atoms, this effect is enhanced.  So while all those extra atoms and light scattering probably isn’t a great thing (::cough, cough::, smog), it does result in some beautiful red sunsets. 

When the sun is low in the sky (at sunrise and sunset), it travels through a lot of atmosphere to get to your eye.  This causes a lot of scattering, and the light that finally gets to your eye has lost a lot of blue, leaving mostly oranges and reds.

This same effect also makes you less likely to get a sunburn early or late in the day.  UV – ultra-violet, which is responsible for sunburn – has an even shorter wavelength than violet, so is scattered more strongly yet. So light that travels through a lot of atmosphere has much of the UV filtered out.  

If you prefer a video explanation of all of this, I recommend "Why is the sky any color?" from PBS’s “It’s Okay to Be Smart” YouTube channel.

You generally need a lot of atmosphere to see the effect of turning white light into blue or red sky.  Undeterred, I thought I’d try it on my kitchen table.  Ingredients: my bike light (white, of course), and a glass tray of water… with just a little bit of milk mixed in.

Simulating the blue sky by replacing the sun and atmosphere with a bike light and some water with a bit of milk mixed in.  The milk acts just like the air molecules, scattering blue light up towards my eyes.

The milk particles floating around in the water act just like the air molecules floating around in the atmosphere, scattering the white light towards my eye, and shifting its color toward the blue end of the spectrum.  Notice that the light that makes it straight through the milky water is shifted towards yellow.

We can also look end-on, and see an effect pretty closely resembling the day-time sky.  Looking directly at the light, it’s still mostly white.  But if you look off to the left and right of the bike light, the water definitely has a blue-ish tinge to it.  And maybe you can convince yourself that there is a yellow-ish haze separating the white light and the blue sky, er, milk water.

When looking end-on at the bike light "sun" through the milk + water "atmosphere", you can actually get a pretty decent replica of the sun-in-the-sky!

I am struck by the juxtaposition of this perhaps most notorious of all science questions – “why is the sky blue” – with its fairly sophisticated explanation, requiring understanding of atoms and how they interact with light waves.  While the question is often associated with child-like curiosity, the answer did not come until I had conquered college-level Electricity and Magnetism and Optics.  There is no question that is too simple to be worth asking.  When we let curiosity take hold, and allow science to illuminate the way, great insights and new ways to see the world – and the sky – are sure to follow.  My new project at DonorsChoose.org/sciathlon – “I Can See Clearly Now” – will help 5^th graders in Washington discover the fascinating and often un-noticed behavior of light and how it interacts with matter.  Here’s to blue skies for all!