NASA says that smoke from forest fires in the Upper Midwest will have a surprising side effect. It’ll bring stunning sunsets to Montana, North and South Dakota, Minnesota, and Wisconsin, as the leftover pollutants drift through their skies.
Pollution is often said to enhance sunsets, most famously in Rome, Italy. “Rome is a dirty city,” says Father Joshua Allen of the Catholic News Agency. “Black soot accumulates everywhere. Emission standards are low, and Rome is surrounded by mountains, so there is nowhere for the dirty air to go. As a result, there are spectacular sunsets regularly.
But is this true? Does pollution really have this silver–or rather gold–lining?
“It is often written that natural and manmade dust and pollution cause colorful sunrises and sunsets,” writes Stephen F. Corfidi for the NOAA/NWS Storm Prediction Center. “However, if it were strictly true that low-level dust and haze were responsible for brilliant sunsets, cities such as New York, Los Angeles, London, and Mexico City would be celebrated for their twilight hues. Clean air is, in fact, the main ingredient common to brightly colored sunrises and sunsets.’
To see how different kinds of pollution change the color of our skies, it helps to know what makes it blue in the first place, and why sunsets are red, clean air or not.
Very small particles, air for instance, scatter the blue wavelengths of light, which is why we have blue skies. When the sun gets low in the sky, like at sunset, its light passes through a lot more air, which scatters more blue light and leaves a higher proportion of red light. This is why the sun turns red, and the evening light glows golden.
So how do the fires in the Upper Midwest cause the spectacular sunsets? Two reasons: first is that the smoke released by the fires is small. “The size of the smoke particles is just right for filtering out other colors meaning that red, pink and orange colors can be seen more vividly in the sky,” says NASA’s Goddard Space Flight Center. The second is that these particles are higher in the atmosphere than the pollution which hangs over our cities.
Imagine seeing the sunset from the “side” of the Earth. As the sun sinks closer to the horizon, we can picture its rays no longer filtering through the pollution layer, but shooting upwards into the sky, where it will be reflected off any clouds up there. If the sky is filled with aerosolized smoke particles, then they turn that light a deep red before it hits the clouds, and the result is ragged layers of scarlet and gold over a deep indigo strip.
So there it is. Local pollution over cities can turn the sunset redder, but only up to a point. Those lingering pollutants need to be of a similar size to air molecules, and they need to be aerosolized, or suspended in that air. That’s what’s happening over Rome. If the pollution gets thicker, or if it’s particles are too big, then it creates a haze which blanks out the spectacle.
“At some point, the air pollution is so bad, and the sky is so saturated, you don’t even see the sun clearly anymore,” Sergey Nizkorodov, of UC Irvine, told Scientific American.
These high-atmosphere particles are also kicked up by volcanoes, and two of the biggest eruptions in modern history caused sunsets that inspired artists. Donald W. Olson, a physics and astronomy professor at Texas State University, concludes that Edvard Munch’s The Scream features such vivid skies because it was inspired by the skies reddened by Krakatoa, which he would have seen a decade before he made the painting (on the other hand, Munch — an expressionist–hardly created photorealistic paintings).
The 1815 eruption of Mount Tambora in Indonesia was so big that its ashes blocked enough sun to blot out the world’s summers in the following year. The cold is said to have inspired Frankenstein, and the skies may have given JMW Turner something to work with.
But that doesn’t mean we want the keep the pollution in our skies. The other side effects of the Mount Tambora eruption were global winter, failed crops, starvation, and lot and lots of death. Climate change, however beautiful, was always bad news.