Why A Volcanic Eruption In Chile Is Turning Brazil’s Sky Purple

Why A Volcanic Eruption In Chile Is Turning Brazil’s Sky Purple

This is what a sunset in Rio de Janeiro looks like right now, and it’s all thanks to that volcano erupting in Chile last week. Calbuco spewed 210 million cubic metres of ash into the atmosphere, turning nearby regions into a “grey desert” and altering weather thousands of miles away.

The massive clouds of of ash that shot out of Calbuco when it first began erupting were composed of dust and sulphur dioxide. The ash is ejected into the stratosphere and spread by wind. Sulphur dioxide reacts with the atmosphere to form sulphate aerosols — fine particles that become suspended in the air.

The ash cloud can be seen originating in southern Chile, then spreading northwards, across Brazil.

The sun’s light is always being scattered and refracted by molecules in the atmosphere on its way to your eyes. The sky looks blue during the day because those molecules scatter indirect light from the sun and sunsets normally appear red and orange because the light has to travel directly through more of the atmosphere to reach your eyes and the longer wavelengths of those colours are what makes it through.

Sulphate aerosols generated by an eruption add more obstacles for the sun’s light, intensifying the red and orange effect by blocking more of the other colours.

So why is Brazil’s sky purple? It could be due to light interacting with both the sulphate aerosol and dust. Atmospheric Optics explains:

“The purple glow is probably a combination of red-orange light transmitted through the lower atmosphere and scattered blue light from still sunlit stratospheric dust.”

The red of the sunset is intensified as it travels through the sulphate directly from the low sun to you, while the upper reaches of the atmosphere are filled with dust, reflecting that blue light downwards.

All these materials on the upper atmosphere can also lead to other novel sunset effects and colours, including bright yellow “twilight arches” and diffuse sun rays and shadows.

Scientists have recently discovered that they can determine the amounts of ash ejected by ancient eruptions by studying historic paintings of sunsets. The red sky in Edvard Munch’s The Scream has been shown to coincide with the atmospheric effects caused by the 1883 eruption of Krakatoa.

It was ash from that eruption that led to the discovery of the jet stream; it enabled scientists to map global weather patterns. In fact, they first named it the “equatorial smoke stream.”

Of course, all that ash doesn’t just create pretty colours. Calbuco blanketed the nearby town of Ensenada with a 50cm thick coating. The BBC describes the regions as a “grey desert”, and the ash is collapsing roofs, destroying grazing for farm animals and closing roads. Should it rain, the resulting mud could lead to destructive landslides. Chile is also one of the world’s top salmon producers and the ash is killing those fish by the thousands. Economic impacts have not yet been totalled, but will likely be billions of dollars.

Calbuco is a small eruption by volcanic standards. That much larger eruption of Krakatoa in 1883 generated far more ash. Enough, in fact, to lower the global temperature by 1.2C in the year following. The sulphur combined with water in clouds to form sulphuric acid, causing widespread acid rain. And, the subsequent year (July 1883 to June 1884) became knowns as the “Water Year” in southern California as over 38 inches of rain fell on Los Angeles; up from an average of 15 inches.

Calbuco’s ash will next be carried across the Atlantic to Africa, then likely onto the Middle East. Its effects will diminish as the materials disperse but, if you’re in its path, you have one last visual effect to look forward to. A bluish halo will form around the moon and brighter stars. Known as a “Bishop’s Ring,” it’s caused by the ash refracting their light.

Picture: Helio de Carvalho Vital