Why is the sky blue? It tends to be one of the most common people of one of the most present elements of nature. It is a curious doubt considering that the sky has multitudes of colors depending on a series of conditions such as weather or time.

Yes, the sky is blue during part of the day, but very early in the morning it can be a very light blue, orange and red during sunset and dark gray during the night. The shade of blue is even different at different points on the planet and is not only related to location but also to altitude or weather phenomena.

But the question remains: why is the sky blue? Especially if we take into account that the sun, which provides light to the planet, is a light yellow color that makes you blind if you look at it directly. But all this has a simple scientific explanation.

To answer the question we first have to remember how the atmosphere surrounding planet Earth is composed. It is a mixture of:

  • 78% nitrogen
  • 21% oxygen
  • 1% argon and water vapor

In addition, there are suspended dust particles, ice crystals, trace amounts of other gases. We also have to take one more thing into consideration: the density of the atmosphere changes depending on the altitude. And as we explained before: that affects the color of the sky.

Light rays, in space, in a total vacuum, travel in a straight line, but once they enter the Earth’s atmosphere they encounter these elements, causing the behavior to change.

The way in which its behavior is modified will depend – in a simple and straightforward way – on two things: the wavelength of the light and the elements with which it meets.

Sunlight, in fact, is an electromagnetic wave with different wavelengths, some of which we can see, corresponding to red, orange, yellow, green, blue and violet.

So, if the dust particles and/or water droplets are considerably larger than the wavelength of the light, they will act as mirrors and reflect the light in different directions, but without changing its color.

But if the molecules suspended in the atmosphere are smaller than the wavelength of the light photons, they absorb the light and re-emit it in different directions due to the electrical polarization of the particles.

This process is known as scattering. Particles in the atmosphere are especially efficient at scattering short wavelength light, i.e., the light we see in blue.

That is why the sky as we see it during the day is blue. This process was first explained by John William Strutt, third Baron Rayleigh, in 1871 and is therefore known as Rayleigh scattering.

Rayleigh scattering also explains why the sky is red and not blue at sunset. This is because, as the Sun gets closer to the horizon and the Earth’s surface, the atmosphere is denser.

That is why the rays that reach our eyes are already depleted of short wavelength photons, corresponding to the color blue. But the remaining waves, those that do reach us, correspond to longer and less scattered length.

And this is, in short, the explanation why we have such beautiful sunsets and a spectacular blue sky.


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