There’s no way to say for sure what’s behind this lavender curtain, but we have a guess: Raikoke. It’s a Russian volcano that erupted June 22 this year. Nobody was hurt — the volcano sits on its own uninhabited island along the Kuril chain draped along the Sea of Okhotsk in the northwest Pacific. But that didn’t prevent it from injecting aerosols — including sulfur dioxide — some 43,000 to 52,000 feet high, into the stratosphere. That’s the layer of atmosphere above which most weather happens, the troposphere.
“Researchers monitoring the stratosphere with high-altitude balloons soon found a layer of sulfur particles 20 times thicker than normal,” Science magazine reported.
You can see the sulfuric aerosol injection in this satellite plot from NASA. Note the blob of red engulfed within an extratropical cyclone bowling through the Bering Sea toward the Aleutian chain. (The absence of sulfur dioxide pixels west of a vertical line is probably just the result of the time that sector was scanned by the satellite, which probably took place just before the eruption.)
Shortly thereafter, you can see the material becoming entrained by the jet stream, pooling in troughs or low pressure zones at the mid-latitudes. By August, it was beginning to thin, and once September rolled around, the aerosol concentrations had become significantly more diffuse. The fact that the satellite no longer is registering the aerosols as pixels doesn’t mean they’re gone — instead, that material has been spread much thinner in the upper atmosphere to form a fragile veil.
These sulfuric aerosols are known to scatter blue light. Sunsets are already red, so when you mix the two colors, you get purple.
Tony Phillips of Spaceweather.com wrote about a number of “volcanic sunset” sightings across the United States and the Northern Hemisphere in early August. (While Anak Krakatau produced a slightly larger eruption in Indonesia shortly before Christmas, it would take close to, if not more than, a year for stratospheric material from the Southern Hemisphere to make it across the equator in the Northern Hemisphere.) Les Cowley, creator of the website Atmospheric Optics, writes that the “finer dust particles remain aloft for years and spread around the world.”
There may be a tendency for slightly denser concentrations of the aerosol to gather in the wake of sweeping cold fronts.
The largely invisible blanket of fine aerosols in the upper atmosphere may also serve as a backdrop on which to catch sunlight; that could help make for brighter, more vibrant sunsets.
Have you seen a sky like this lately? Send us your pictures; we’d love to see them!