Why Is The Sky Blue?

A gorgeous blue sky is something most of us enjoy and admire, but why is the sky blue? I remember many years ago in the spring it was an unusually cool day and the sky overhead was the deepest blue that I had ever seen. Never before or since then have I ever seen a sky that blue, it really sticks in my memory. The short answer to this question is that the electronic structure of oxygen is such that the visible portion of the electromagnetic spectrum(light) is absorbed by the oxygen molecules except for the blue portion of the spectrum in a process known as Rayleigh scattering, thus giving us our blue sky. Of course, when I give this answer I am referring only to the way most humans perceive colors – there is some variance among humans and a small percentage can not see color at all. And most animals will perceive colors quite differently than humans, if at all.

To get more insight as to why the sky is blue, lets first look at the composition of the Earth’s atmosphere. Most of us know that the atmosphere has oxygen, but what other constituents are there and in what proportion? The Earths atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 1 percent Argon, and various trace amounts of carbon dioxide, neon, helium, methane, krypton, hydrogen, nitrous oxide, xenon, ozone, iodine, carbon monoxide, ammonia, and water vapor in the lower part of the atmosphere. Although there is about 3.7 times as much nitrogen in the atmosphere as oxygen, it has no significant effect on the color of the sky under normal circumstances. Under some conditions, nitrogen oxides can form which will give a somewhat brownish hue to the atmosphere which is commonly called smog. Thus oxygen is the molecule which has the dominant effect during the Rayleigh scattering process on the color of the sky. It is important to note that many other atmospheric conditions can affect the color of the sky – weather, the angle of the sun, the amount of water vapor in the atmosphere, pollution, and so on. For example, when the Sun is high in the sky there is less atmosphere for the light to pass through and the sky will have a more blueish color; when the Sun is low on the horizon its light must pass through a much thicker layer of atmosphere and sky will have a pale blue or even reddish color.

Diffuse sky radiation, also known as skylight, simply means that solar rays hit the Earths atmosphere and are scattered by various molecules and particles. Roughly two-thirds of the solar light hitting the atmosphere reaches the Earth as diffuse sky radiation and is responsible for changes in the sky color. The basic diffuse sky radiation scattering effects are Raleigh Scattering, explained in detail below, and Mie scattering, which is a derivation from Maxwell’s electromagnetic equations that explain the scattering of a plane type of electromagnetic wave, in this case visible light, from a homogenous sphere. Both of these processes are elastic which simply means that the light(electromagnetic) waves can change their direction from the scattering effect without being absorbed and with no change in wavelength.

Short wavelengths of light are scattered in the atmosphere(sky) much more than the longer wavelengths – since blue is a significantly shorter wavelength than red, for example, it is scattered much more predominantly across the sky, so on a sunny day we have our blue color. When the sky is overcast, essentially all of the light reaching the Earth is diffuse sky radiation; this occurs because the size of the water droplights in the clouds is larger than any of the wavelengths of light, so the different wavelengths are spread across the sky pretty much uniformly. The intensity of the light on an overcast day is also considerably less than on a sunny day. On a light overcast day, it is around one-sixth that of a clear sunny day. But on a really heavy storm day, the intensity of light can be as little as one-thousandth!

A word should probably be mentioned about smog, either natural or manmade from atmospheric pollutants. The particulates in the atmosphere in such a case will also lower the intensity of the light reaching the Earth; the heavier the pollution/smog the less light that will reach the Earth’s surface. Similar to the situation with natural clouds in the atmosphere, different wavelengths of light will be scattered more uniformly so there is less variation in colors and the sky will appear more grayish.

Rayleigh Scattering

Raleigh scattering is the scattering of electromagnetic radiation, in this case the visible light spectrum, by particles which are significantly smaller than the wavelengths of the radiation, in effect atmospheric gas molecules. Please note that in particle physics this is known as elastic scattering which is a type of scattering where the kinetic energy of a particle is conserved in the central mass, here a gas molecule, but its direction of propagation is then changed by the interaction with other particles. Raleigh scattering combined with the previously mentioned effect of the electronic structure of the oxygen molecule where shorter wavelengths of the electromagnetic spectrum are absorbed and longer wavelengths such as blue light are scattered give us our blue sky. The sky overhead will generally look bluer than the sky near the horizon since the scattered blue light has more atmosphere to travel through and so more of it is scattered away in different directions. Thus when an observer looks directly overhead the sky will appear a deeper blue and when the observer looks toward the horizon the sky will appear a much paler blue.

It should be noted here that about 20.8 percent of the oxygen in Earths atmosphere is diatomic, that is 2 atoms of oxygen joined together, while ozone, which is triatomic oxygen(3 atoms joined together), occurs in only very small amounts, about .00006 percent and the vast majority of that(about 97 percent) is in a range of about 9.3 miles to 34.2 miles above the Earths surface in the stratosphere. This stratospheric ozone is very beneficial to life on Earth since it absorbs harmful ultraviolet rays. But ozone at ground level is very harmful to breathe and can cause permanent biological damage since it is a much more reactive molecule than the diatomic oxygen which is predominant in the atmosphere.

So now we know what causes that gorgeous deep blue sky that we often enjoy so much. It is basically the Raleigh and Mie scattering effects combined with the fact that atmospheric gases and particulates, especially oxygen, absorb the shorter wavelengths of light while letting the longer wavelengths, blue light, scatter to various degrees in Earths atmosphere. When looking overhead through the thinner layer of atmosphere the sky appears bluer, when looking toward the horizon through the thickest layers of the atmosphere it appears a paler blue or maybe even white. For example, at sunrise or sunset, the sky will have a paler blue color and the Sun will appear much more reddish in color. And when we have overcast days either manmade or natural, the sky has a somewhat bland white or grayish color. The temperature of the atmosphere can also affect how deep the blue color is as well.

The Earth is so very fortunate to be in just the right orbit at the perfect distance from the Sun to develop and sustain intelligent life – life forms that are able to appreciate wonderful blue skies on sunny summer days. So go ahead and enjoy those fantastic blue skies the next time you are outside with even more appreciation now that you know what causes it.