Why do the clouds look red?

Every day the sunrise and sunset gives us an incredible spectacle. Sometimes it seems that the sky has caught fire or that a great volcano is burning on the horizon, but what is really happening is that physics is playing with our sensibilities.

Why is this happening? The cause lies in a physical phenomenon that occurs because the white light that comes from the Sun is actually made up of a mixture of colors, as can be seen in the Pink Floyd album covers or in a rainbow. The reason is that each of the colors has a different "easiness" to pass through the atmosphere. So, if at sunrise and sunset there are more orange or red colors, it is because the blues and purples have spread more through the sky and have not reached us. Finally, if the clouds look red, it is because they reflect the light as it reaches them: for example, white at noon, orange and red in the afternoon.

Why do we see colors?

It must be borne in mind that colors are nothing more than human perceptions of a physical phenomenon. Each of them is actually defined by a parameter called wavelength. This is one of the properties that explain how a wave moves, and which is also closely related to the amount of energy it carries. If we wanted to define the wavelength, we could think of the ripples that appear in a pond if a stone is thrown: the wavelength is the distance between two consecutive crests (waves).

What does this have to do with colors? Much. Basically, colors are interpretations of the human eye and brain to represent the wavelengths of light. If the "waves" are very close together, the eye interprets that we are seeing blue light. If they are further apart, go orange. If they are even further apart, red comes into play. In fact, there are waves so tight or so separated that the eye is not capable of detecting them, and that is when it is said that they form part of the non-visible spectrum of electromagnetic radiation (in there is ultraviolet light, which some insects do can see, or the invisible radio waves).

Therefore, now that we know what colors are, we can understand that if the sky looks blue it is because light reaches us with a wavelength that we interpret as blue. But if you see red, it is because the corresponding wavelength mostly arrives.

Now you have to wonder why the sky is sometimes "enriched" in some wavelengths and other times by others. Furthermore, we have said that light from the Sun is white! How can there be so many colors?

The reason is that the light that comes from the Sun is made up of a very diverse mixture of waves that have different wavelengths. This can be seen when light passes through a prism or a cloud of water drops: a rainbow is then formed, in which the waves are clearly separated. If we could "catch" these beams, and divert them so that they join again, we could obtain a new white beam.

Now we know that: A) Colors are our brain's interpretations of the wavelengths of light. B) White light is a mixture of various colors. It's time to hit a beam of light against the atmosphere and see what happens.

Light is bent in the atmosphere 

First of all, you have to understand that photons don't just pass through the atmosphere. Just as a passenger may have trouble navigating a crowded subway station, for photons to pass through the atmosphere they have to find their way between gas molecules in the atmosphere. Obviously, they will collide more and have more problems the denser it is.

This has several consequences. First, some of the light is dissipated as heat (through a physical process known as absorption). Secondly, another part of the energy is redirected (scattered) in secondary waves: this is similar to what happens if we aim the pressure stream of a hose against a wall, so that the water is deflected in drops and different jets. In the case of light, this happens millions of times along its path through the atmosphere.

The composition of the atmosphere must also be taken into account. Each atom and molecule has a different capacity to absorb and scatter light: in fact, each of them will absorb and scatter some wavelengths more than others. For example, thanks to this, ozone is a gas that protects from solar radiation: it is "specialized" in stopping the wavelengths that correspond to ultraviolet light, and thanks to that it becomes a fantastic shield against this harmful radiation. However, ozone does not filter yellow light, for example.

In general, the phenomenon that explains this type of light scattering is known as Rayleigh scattering. It occurs when a ray of light is scattered by particles that happen to be smaller than its wavelength (the space between the crests of the wave). This issue of size is important because if the particles were larger than the wavelength, their way of scattering light would be different. In any case, what is more important is that Rayleigh scattering is the reason why the sky turns blue or red.

Blue or red thanks to Rayleigh

Why? During the day, the oxygen and nitrogen that make up most of the atmosphere scatter light, (that hose stream deflects and scatters as it collides with atmospheric molecules). But there is something else. According to Rayleigh scattering, the light that is most efficiently scattered in the sky is the light with the shortest wavelength: that is, blue. On the other hand, red does too, but much less frequently. The consequence is that most of the scattered light is blue, while the rest continues on its way. And because of that, the sky is dyed blue.

If that happens, it is because of the composition of the atmosphere. For example, if we fly to higher layers, the sky looks dark blue and purple because these are the most scattered wavelengths at that altitude. If we add suspended dust to the mix further down, the sky will look more brown or yellowish.

During sunsets and sunrises something different happens. The Sun is very low on the horizon, and its rays of light pass through a long stretch of the densest atmosphere: that which is closest to the ground. And not only that: its path is also longer than when the Sun is at noon. This increases the probability that blue light will scatter before reaching the ground or clouds. For this reason, the mixture of colors is enriched in yellows, first. When these disperse, orange predominates. If it disperses, the mixture turns red.

And what about the clouds?

Now we need to understand why clouds look the way they do. The cause is not Rayleigh scattering, but Mie scattering. This occurs when the scattering particles are larger than or equal to the wavelength of light. In the Earth's atmosphere, this type of encounter occurs when there are small droplets of water in suspension, condensed in the form of mist or clouds.

Unlike the Rayleigh effect, Mie scattering does not distinguish between the wavelengths of colors. Disperse all of them equally. Therefore, the only thing that clouds do is reflect the light that reaches them. If the light that arrives is white, the clouds are white. If it's orange, they'll look orange. However, clouds are not always white. Why?

It can happen for two reasons. When the clouds are light, the blue background of the sky can make them appear darker, turning gray. It can also happen that the background clouds and sky are so bright that the clouds in front appear darker.

If the clouds are very dense, they reflect the light perfectly and appear white. If they are less dense, some of the light is absorbed and they appear black.

Pollution and why the sky is not purple

And finally, a couple of curiosities. The most spectacular and brightest sunsets and sunrises can be seen when we travel on an airplane. Why? Because we are moving away from the densest layer of atmosphere, and normally full of dust and pollution. If we still want to enjoy sunrises and sunsets at ground level, we must choose the days when the atmosphere is drier and cleaner.

Another thing. If the maximum sensitivity of our eyes did not occur near the green wavelength, we would see the sky as purple. Would we say that Earth is the purple planet? Or would we call that purple blue and blue purple?