Why the sky turns red at sunset

When sunlight enters the atmosphere it meets molecules of nitrogen and oxygen far smaller than the wavelength of light. They scatter light in every direction, but not evenly: the shorter the wavelength, the more it is scattered. Blue is scattered much more than red, so wherever you look in the sky, away from the sun, blue light reaches your eyes from all sides.

Violet is scattered even more than blue, so why is the sky not violet? Because sunlight carries less violet than blue, and our eyes are more sensitive to blue. That is also why the sky looks paler near the horizon: light from there has crossed more air, and part of its blue has been scattered away again.^{[1, 2]}

It is the same process, seen from the other side. With the sun low on the horizon, its light no longer comes down almost straight through the atmosphere: it crosses it sideways, along a path many times longer. By the time it reaches us, most of the blue and violet has been scattered out of the way, and the light that is left is yellow, orange and red.

That is why a sunset sky is not one color but a gradient: warm and red near the horizon, where the light has travelled furthest through the air, and still blue overhead. Every sky palette on Pigmentoria is read in that order, in six bands from the horizon to the top of the sky.^{[1, 3]}

Air is not only gas. Larger particles of dust, pollution and water vapor also scatter light, and they scatter more of the reds and yellows, which can make the whole western sky glow red. The more particles there are, the more scattering, and the more striking the sunrise or sunset can be.

The strongest cases come after a great volcanic eruption, when ash and gas reach high into the atmosphere and spread around the world. After the eruption of Tambora, in Indonesia, in 1815, painters in Europe saw the colors of the sky change, with red and orange sunsets for up to three years.^{[2, 3, 4]}

The English saying «Red sky at night, shepherd’s delight; red sky in the morning, shepherd’s warning» goes back to the Gospel of Matthew. The Met Office explains why it often holds: a red sky appears when dust and small particles are trapped in the air by high pressure, which scatters the blue and leaves the red. In the evening the sun sets in the west, so a red sunset means that high pressure, and dry weather, is arriving from the west. A red sunrise, in the east, means it has already passed, and a wetter, windier low may follow.

It only works where weather usually arrives from the west, as it does in Britain and much of western Europe. It is a rule of thumb from centuries of looking at the sky, not a forecast.^[5]

J. M. W. Turner was one of the painters who recorded the blazing sunsets that followed Tambora. In 2007 a team of atmospheric scientists led by Christos Zerefos measured the ratio of red to green in the sunsets of a few hundred paintings made between 1500 and 1900, and found that it rose after great eruptions: it matched the records of volcanic dust in the air.

In 2014 the same team went further, with hundreds more paintings and a test in the open air: a painter painted successive sunsets on the Greek island of Hydra while dust from the Sahara passed over it, and the reds in his pictures matched what the instruments measured. Paintings are not instruments, but the most attentive painters saw the color of the air with real accuracy.^{[4, 6, 7]}

No two sunsets are the same: the sky changes with the season, the weather, the humidity and the dust in the air, from one evening to the next. Each sky palette on Pigmentoria is our own reading of freely licensed photographs of the place: one evening or one morning, taken as inspiration. They are a guide for looking, not a measurement of the sky.