Sunset Skies and the Science.

Why a Sunset Happens

To most of us this is a brilliant sunset.

To Christos Zerefos, however, it is an \"intense optical phenomenon\", a priceless piece of scientific evidence for working out the world\'s climatic history. And it is just one of more than 500 paintings by famous artists that a team led by the Greek professor has been analysing to work out the impact of giant volcanic eruptions.

Where the rest of us see beauty in a fine sunset or sunrise, atmospheric scientists see dirt and dust. For these particles help daub the brilliant reds and golds of these skies.

Evenings and mornings take on these hues, rather than the blue of the day, because the slanting sunlight has further to go through the atmosphere before it reaches our eyes. The longer it has to travel through the atmosphere, the more it is scattered by the molecules of the air. The shorter wavelengths – violet, blue and green – are dispersed the most, leaving yellow, orange and red to get through.

Particles – and tiny droplets of sulphuric acid – magnify the process, and volcanoes, which throw vast quantities of both up into the stratosphere, have long been known to produce amazing sunsets. The giant eruption of Krakatoa in 1883 produced particularly brilliant sunsets, as did that of Mount Pinatubo in the Philippines in 1991.

And both the dust and the droplets also help to cool the Earth by bouncing the sun\'s rays back into space; in recent years they have helped to offset global warming.

This is where Professor Zerefos, of the National Observatory of Athens, and his team came in. They reckoned that paintings of sunsets before and after volcanic eruptions could help to show how much these had affected the climate of their times.

So they studied 554 pictures depicting sunsets, produced between 1500 and 1900 by 181 artists, including Rubens, Rembrandt, Reynolds and Renoir – taking images from the websites of 109 galleries and museums.

Five artists – Copley, Turner, David, Ascroft and Degas – were particularly useful, as they depicted different sunsets before, during and after major volcanic eruptions. Turner was in a class of his own because he painted so many – in all, 115 of the studied canvasses were his – and his career spanned three such events, in 1813, 1831 and 1835.

Sure enough, the scientists found that the artists had faithfully reproduced what they saw – or, as their paper, just published in the journal Atmospheric Chemistry and Physics puts it, they \"appear to have simulated the colours of nature with a remarkably precise coloration\".

The sunsets they painted were reddest in the three years after major eruptions, and confirmed what was already known from other records about their effects.

Professor Zerefos\'s team is now hoping to study 40 works in the Tate, painted in the last century, to see what they can tell us about air pollution at the time – a trickier task as the effects of man-made dirt on sunsets are less straightforward than those of volcanoes.

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