Why Is the Sky Blue If Air Is Invisible?

White heap clouds in the blue sky on a summer day.

If you look up in the sky, depending on the time of day, it changes color. In the early morning and twilight evening, it has shades of orange, pink and red. However, during a good part of the normal daylight, the sky tends to be different shades of light blue. And, when the sunlight is really strong, it seems almost not blue at all, maybe even white with the glare.

Aside from modern air pollution, which creates a bland gray haze, the sky has pretty much been the same color spectrum for centuries. Only the occasional storm or a volcano has had a dramatic effect on specific regions.

So, exactly what makes the sky blue then if air up close has no substance, color, texture or shade? Much of the answer depends on how air molecules capture the light and distribute it.

Light from the sun hits the earth as brightness first off. In that light is an entire rainbow spectrum of color. This was proven by early experiments like those created by Isaac Newton and similar. Using a prism, they were able to show that basic clear light could actually be changed into a rainbow of colors if the light hit the prism the right way and was then bent as it passed through to the other side.

Pink sky during sunrise over water.

For our human eyes, light passing through air molecules has a similar effect. The air molecules catch the light and scatter it so that certain colors are seen at certain times, depending on the angle of the light arriving and the position of the air molecules.

During the main part of the day, blue light is scattered while red light is passed through fairly straightforwardly. In the early morning and evening, however, it is the red and orange light that is seen because the blue light is reflected out and away from our path of sight on the ground.

There Is Light That Exists and Then the Light We See

The first factor is the speed of light. From a physics perspective, the color difference occurs based on how fast the light energy travels. Each color within the “white” combination of light has a different wavelength in terms of its travel behavior. Slower and faster wavelengths produce different colors.

What we see with our human eyes isn’t the entire electromagnetic spectrum that exists, but a portion that sits between 720 nm to about 380 nm. Our eyes “see” these colors because of the receptors inside the structure of the human eye and how it interprets the colors as the light is passing through our range of vision.

Visible spectrum of light with color wavelengths.

Does this mean that a horse or a dog sees colors differently than a human? Absolutely. In fact, because of the eye design in different species, we as humans may very well see a greater range of wavelength differences than some animals that only see differentiation of heat and movement versus a full rainbow spectrum. However, some animals may see more colors than humans.

It’s Our Fault as Well

The second influencing factor comes from us as viewers. Our perception of color is highly influenced by our own eyes, but the light wavelength is always present, whether we see it or not.

For those who are colorblind, for example, their eyes see some colors but not others because their receptors are working differently than the average human eye. While they may very well see a blue sky above, they may not see green or red on the ground. The reason, again, has to do with how their individual eyes perceive light and then translate it to the brain.

There’s Another Factor Involved

The third element in the color of the sky specifically has to do with the air molecules themselves. As discussed above, light is already a combination of light energy itself traveling at certain wavelengths and how our eyes interpret what we see with receptors. Air molecules influence things further by scattering light in certain concentrations.

The scattering effect was first defined by John Tyndall in the mid-19th century. Dubbed the Tyndall Effect, the process refers to how air molecules allow some types of light through while reflecting or misdirecting other aspects of light. Doing so, air molecules impact the type of light that makes it to the ground where the viewer stands and is looking up.

John Tyndall’s work was refined by Lord Rayleigh some years after the fact, so the process is also known as the Rayleigh Scattering as well, depending on whom people want to give credit to.

Tyndall effect in a forest.

It is not moisture in the air that triggers the blue coloring that we perceive. While air molecules are definitely impacting the refraction of light, and our eyes are receiving the effect on the ground depending on where we are located, humidity or moisture in the air is not a fourth influence producing bluish views.

Instead, Albert Einstein concluded a decade into the 20th century that it was actually the electromagnetic nature of light that caused a reaction in air molecules it hit, causing them in turn to reflect light in such a way that we see blue from the ground looking up during midday.

First off, the upper atmosphere does a very serious job in protecting the earth. A good amount of light doesn’t make its way through as a result. For example, if you were out in space, the sun would be a blazing source of white light, unfiltered by anything when looked at directly. However, when the sun’s light hits the atmosphere, the light spectrum that makes up violet tends to fall apart and not make it through very well. By the time the light then gets to the surface, where we are, our own eyes don’t favor violet at all.

In fact, the human eye tends to favor red, green, and blue, seeing different colors as a mixture of those three primary colors. The receptors send off a translation to the brain when they see the color they prefer out of the three, and tell us the sky is blue or red.

While this sounds like the eye receptors are a finicky bunch, it’s really because they are designed to be sensitive to certain colors. Green and red receptor cones, for example, pick up yellow and green. Blue cones clearly pick up the wavelengths producing blue. Our eyes’ blue and red receptor cones pick up the slight remaining violet that comes through, which in the twilight gives the purple and reddish hues otherwise overwhelmed during the regular part of the day.

Explaining the Rich Color of Sunsets

Colorful sunset over ocean.

There is no question that when you are looking at the sunset, or early sunrise for that matter, you are looking through far more of the atmosphere than at any other time of the day when viewing light. A significant amount of matter and molecules between the light source and the viewer results, and that mass produces a far greater amount of light scattering than occurs at any other part of the day.

Add in the fact that modern air has a far greater amount of pollution in it than ever before, and the reddish hues of the sunset become pronounced and far greater than they ever were maybe 200 years ago before industrialization occurred.

This varies to some extent, depending on where you are. On the ocean, the salt in the air from the seawater churning is going to create more orange than deep red. However, in both cases, blue practically gets destroyed by the large amount of molecules and matter in the air that it must go through at the sunset or sunrise angle.

Once the angle of view goes higher, though, the blue particles increase and begin to take over, as they are far stronger at that point. Red and violet become scattered and fewer in number, practically disappearing from our eye receptors.

What Will the Sky Look Like in the Future?

Smog in Los Angeles.

A lot depends on the level of pollution that ends up being produced in decades to come. The high part of the day without disturbing weather will likely still remain blue, assuming you can see through the lower pollution haze that could be produced.

This is not a Hollywood movie scenario. London in the early 20th century suffered from severe smog due to pollution, and China in recent years has had weeks on end of severe hazy smog graying out the sky as well. Without a strenuous effort to keep such pollution out of the air, the likelihood of seeing blue skies from an urban location will continue to diminish.

On the other hand, the same pollution will be a significant contributor to very reddish skies during the sunset and early morning. With so much material in the sky, urban areas will be bathed in deep orange and reddish colors for a brief part of the day before reverting back to the sky-choking smog so common in polluted areas. Again, this future is not imagined; it happens internationally in locations where air filtration and regulation are non-existent or poor.

Of course, none of the above may matter much if we or our descendants have to wear masks to breathe at all. Sky color may be an afterthought then, as people spend more time trying to find fresh air to breathe or become wholly dependent on air filtration. Hopefully, things won’t turn out so bad, but the thought alone makes you appreciate the sky’s beautiful colors more.