bright skies named colour of the year – here’s why there’s so much mor…

The colour of 2022 will be “bright skies”, according to paint manufacturer Dulux.

This mellow light blue may certainly seem familiar. Depending on where and at what time of the day you look at the sky, you might well expect to catch a glimpse of a similar colour.

however take the time to watch the sky from the horizon to the area above your head, during all weathers and from dawn to nighttime, and of course you’ll see that it is filled with many colours. Over hundreds of years, physicists have worked to understand why the sky holds so many shades, from a myriad blues to red and already green. Here’s what we’ve learned, and what to look out for while contemplating “bright skies” and immersing yourself in skywatching.

Dulux colour bright Skies compared against a clear blue sky near the horizon with hardly any water vapour (left) and a cloudier sky suggesting higher levels of water vapour (right)
Daniel Brown, Author provided

The Sun’s light is made up of different electromagnetic groups, and their various wavelengths are associated with a different colour. Shorter groups are seen as blue, slightly longer groups as yellow, and already longer as red.

When these groups are seen together they look white. But this light has to travel by our air before it gets to our eyes, and atmospheric molecules are much smaller than the wavelength of the Sun’s light. As the light hits these molecules, they scatter it in all different directions. This effect is called Rayleigh scattering.

In this course of action, more of the bluer light, which has shorter wavelengths, is distributed, resulting in the sky becoming blue wherever you look. Meanwhile, the Sun becomes more yellow looking since the light from it is now missing those longer blue wavelengths.

The sky seen directly above and explored from within the Deer Shelter Skyscape installation by James Turrell in the Yorkshire Sculpture Park.
Author provided

Adding white

But the daytime sky isn’t the same blue all over. You’re more likely to find the Dulux bright skies colour closer to the horizon where the blue is more washed out or lighter.

This is the impact of Mie scattering, which is a similar course of action as Rayleigh scattering but caused by larger particles (such as water vapour or fine pollution particles in little droplets). These types of particles remove the red, yellow and blue colour elements from a white light beam in equal measures and do not alter the colour of the light passing by the air or being distributed back to an observer. This leads to the sky turning whiter in addition to the blue caused by Rayleigh scattering.

The influence of white within the blue of the sky becomes stronger towards the horizon where the light has to pass by much more air to arrive at the observer. The various tones and shades of blue observed become character’s visualisation of what the air is currently composed of. The whiter it appears, the more additional particles are present.

A tool to measure just how many particles are suspended in the sky was developed by Horace Bénédict de Saussure, an 18th-century Swiss geologist and alpine explorer. Called a cyanometer, it is a colour wheel featuring 53 different colours for the observer to compare to the sky.

A modern version of a Cyanometer.
Daniel Brown, Author provided (no reuse)

Ozone blue at twilight

If you skywatch at dusk, you’ll see a bright characterize of colour that captures intense red tones especially close to the direction of the setting sun. Since the Sun’s evening light travels by much more of our air than when the Sun is higher in the sky, by the time it reaches us it has lost much of its blue part by Rayleigh scattering. If aerosols are present higher up in the air – for example caused by volcanic eruptions – this can become far more extended and colourful.

Once the sun is below the horizon, you will see a strong blue colour in the sky again. This cannot fully be explained by Rayleigh or Mie scattering. Instead, this is due to the presence of ozone (a colourless or pale blue gas), which does not scatter the light but absorbs it and breaks it apart.

Its impact is only noticeable when the rays of the sun have to pass by already more air to reach us (like when it travels from beyond the horizon). The ozone then strongly absorbs red and orange light, making the small amount of light we see in the twilight sky blue.

Red and blue-green night

Venture out at night in a place free of light pollution and its orange sky glow and you might notice that, despite the without of sunlight, the nighttime sky is not black at all. Instead, we can sometimes observe what is called air glow, which is our own air radiating a faint light. This is caused by atoms – mainly oxygen and nitrogen – forming molecules at an altitude of 100km-300km.

This glow is always present but usually too faint to see. But it contributes to the sky turning a very dark red or blue-green colour. You can capture it with cameras that are more sensitive than the eye. But at low light levels, our eyes lose their colour vision and merely see a grey blackness.

In these ways, the colours of the sky show us ways light can interact with our air. And by this science, we’ve already learnt how to recognise and analyze signs of life in the skies of planets beyond our solar system (exo-planets) by analysing the light from them. Traces of an air were first measured in 2001 for the exoplanet HD 209458 b – sometimes called Osiris – in the constellation of Pegasus. In 2019, scientists already discovered traces of water in the air of an exoplanet (K2-18 b) that has temperatures that could sustain life as we know it.

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