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Strange green sunsets reported after the 1883 eruption of Krakatoa were probably caused by the large amount and size of sulphur aerosols the volcano injected into the stratosphere.

鈥淗ow can physics or nature create green twilight colours?鈥� says at the University of Greifswald in Germany, who became intrigued after reading descriptions of such sunsets in the months after Krakatoa鈥檚 eruption in Indonesia.

Outside of London, for instance, meteorologist Rollo Russel described 鈥渞emarkable whitish-greenish opalescence above the sun at sunset鈥�. Others also described green sunsets, although von Savigny does not know of any paintings that depict the eerie light. 鈥淲e don鈥檛 know what tinge of green it really was,鈥� he says.

But just knowing the light was green was enough to learn something new. Von Savigny and his colleagues used a model to simulate how light would be scattered, absorbed and transmitted in the atmosphere by different sizes and amounts of the sulphur aerosols Krakatoa sent to the stratosphere.

They found that, in order to produce a green colour for an observer on the ground, the aerosol particles would have to be very large 鈥� between 500 and 700 nanometres across. They would also need to be abundant enough to scatter a large proportion of the incoming light.

This analysis gives us insights we otherwise couldn鈥檛 gather. at the University of Saskatchewan in Canada says information on particle size can鈥檛 be derived from ice cores the way it can be inferred from the sunsets. He says the added detail from this analysis on estimates of the material Krakatoa injected into the atmosphere could help improve our understanding of the eruption鈥檚 effects on the climate.

While it probably won鈥檛 be the last green sunset we see, they are exceedingly rare. Based on ice core records of volcanic eruptions, von Savigny estimates that an eruption large enough to produce green sunsets occurs about once per century.