Why is the sky blue? Reason for weather optical effect and others explained such as rainbows and red sunsets

There are some things in life that we just accept without too much thought. These are universal truths that we take at face value because they are undisputed facts - but we never question why these things are true. One example of such a universal fact is that the sky is blue.

Now, it could be argued that there are times when the sky is grey on rainy days, white on snowy days, and even pink as the sun sets. All of these things would also be true. But, on the whole, the sky is blue.

Why exactly is this though? Plus, what are the reasons behind some other weather optical effects such as rainbows, red sunsets, white clouds and thunder and lightning? The answer to all of these questions lies in science. Here are all of those answers for you, as explained by the Met Office. This article may come in handy the next time you’re putting together a quiz or your child inevitably asks you at least one of these tricky questions.

Why is the sky blue?

The sky appears blue to the human eye (most of the time), although light from the sun appears to be white. This is because light is made up of a spectrum of many different colours. We see this when rainbows appear in the sky, and we’ll be writing more on that later.

When the sun's light reaches the earth's atmosphere it is scattered, or deflected, by the tiny molecules of gas in the air, mostly nitrogen and oxygen. These molecules are much smaller than the wavelength of visible light and because of this the amount of scattering depends on the wavelength. This effect is called Rayleigh scattering, named after Lord Rayleigh who discovered it.

The shorter wavelengths, which are violet and blue, are scattered the most strongly, and as a result of this more of the blue light is scattered towards our eyes than the other colours. You may then question why the sky doesn't appear to be purple since the violet light is also scattered as strongly as blue light. The answer to this is because the amount of violet in sunlight is smaller than that of blue light to begin with, and in addition our eyes are much more sensitive to blue. As a result of this, the blue light is more visible.

The blue light that gives the sky its colour is also sufficiently bright which makes all the stars that we see at night disappear since the light they emit is much dimmer.

Why do rainbows happen?

A rainbow is an arc of multi-coloured light which appears in the sky when it’s raining but also sunny at the same time. They are created as a result of refraction and reflection. When rays of light from the sun, which as previously mentioned is made up of many different colours, hit water droplets this reflects some of the light back towards the earth. 

Since light travels more slowly through water than air, the light is bent as it enters the raindrop and becomes refracted, splitting the light into the spectrum of colours; red, orange, yellow, green, blue, violet, but in reverse order. Some of the light is reflected off the internal surface at the back of the raindrop, which then becomes a mirror and reverses the order of the colours to provide the familiar sequence of a rainbow we are all used to seeing in the sky. To see a rainbow, you must have the sun shining behind you and the water droplets in front of you. 

Why are sunrises and sunsets red, pink or orange?

Red is one of the longer wavelengths in the previously discussed light spectrum. At sunrise and sunset, the sun is very low in the sky and that means the sunlight we see has travelled through a much thicker amount of atmosphere at these times. Blue light, as previously mentioned, is scattered more strongly by the atmosphere and therefore tends to travel shorter distances and be scattered several times and deflected away in other directions before it gets back to earth. 

As the sunlight passes over a greater distance at sunrise or sunset, however, we actually see the longer wavelength red light and also yellow light. The combination of these two colours means sunsets are also sometimes perceived as pink or orange. This is another effect of Rayleigh scattering.

Why are the clouds white?

Put simply, clouds are white because light from the sun is white. In a cloud, sunlight is scattered equally, meaning that the light continues to remain white and appear so to the human eye, and this gives clouds their distinctive white appearance.

Clouds can also sometimes appear grey, however, as a result of the same scattering of light that makes them mostly appear to be white. When light is scattered in a cloud it is usually sent upwards or out to the sides, which then makes the tops and sides of the cloud look whiter than the base which receives less light. This can make clouds look grey at times, especially at the base and when the top of the cloud can’t be seen from earth. The tops of clouds will always be white, however, as they have a constant source of white light.

At sunrise or sunset, clouds may appear red or orange in colour. This happens for the same reason that the sunrises and sunsets themselves are these colours, as explained above.

Why does thunder and lightning happen?

Thunder and lightning occur during storms, when warm air sits underneath much colder air, causing an unstable atmosphere. Thunder is the sound produced by the rapid heating of air by a lightning strike. It is referred to commonly as a thunder clap because of the loud and sudden sound it creates. When lightning strikes, the narrow channel of air through which it travels reaches temperatures of up to 30,000C almost instantly. This intense heat causes the air to rapidly expand outward into the cooler air surrounding it which creates a rippling shockwave. This is what we hear as a rumbling thunder clap sound.