A Blush of Volcanic Ash

A delicate pink-purple flush in the evening twilight.

A delicate pink-purple flush in the evening twilight.

The clouds cleared in the afternoon and we were treated to a fine sunset. I looked out the window at the right time to see a delicate pink-purple flush above the western horizon in the deepening twilight. Quickly grabbing the camera I snapped a couple of shots just before it faded, which you can see above. A clear dawn also allowed me to grab a couple more the next morning! It confirmed something that I had been wondering about – the volcanic ash from the eruption of Mt. Calbuco in Chile had indeed circled the southern hemisphere.

A stronger purple colour caught before dawn the next morning.

And a stronger purple colour caught before dawn the next morning.

This volcano erupted in spectacular fashion on April 22nd, with two smaller eruptions on the 24th and 30th. While the ash cloud disrupted local flights in South America, it wasn’t enough to stop flights in other parts of the world. The dispersing ash caused brilliant sunsets in Brazil, then South Africa – and then not much else was appeared in the media except for a mention the ash had now circled the globe.

Normally we see some colour at sunset as the light is bent through the atmosphere, blue light being bent upwards and the redder end of the spectrum being bent in towards us. But when there is significant amount of volcanic dust and aerosols present in the upper atmosphere this refractive effect is enhanced.

This relatively thin layer of ash is just giving an interesting tinge to our sunsets, nothing like the vivid red colours that were seen after Mt Pinatubo in 1991, or the famously lurid sunsets associated with the last eruption of Krakatoa in 1883. Mt Pinatubo was the last big eruption to have any significant effect on world climate. As well as the red sunsets the sky appeared more opaque than usual and global temperatures dropped 0.5 degrees for the following two years, and I clearly recall seeing the Moon during the total lunar eclipse of 4th June 1993 as a dark grey rather than its usual coppery-red.

History is littered with the aftermath of big eruptions. 1816 was known as the “Year without summer” in the northern hemisphere after the eruption of Mt Tambora the previous year. Crop failures were common and famine was widespread. Mt Pinatubo’s effects were minor compared to this event. But this won’t happen this time – the ash and dust from Mt Calbuco is far too thin, it’s only going to give us some memorable sunsets, and sunrises, for a while until it disperses.

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