A rash of unusually colorful sunsets follows in the wake of two major volcanic eruptions.
Several evenings ago I noticed an intense purple-pink glow in the western sky about 15 minutes past sunset. I hadn’t seen that color in a long, long time. Could a volcanic eruption be the cause? Volcanoes are known to seed the stratosphere with ash that intensifies sunset colors.
Checking, I learned that the Raikoke volcano in the Kuril Islands and the Ulawun volcano in Papua New Guinea each had blasted gases and dust more than 18 km (11 miles) high on June 22nd and August 3rd, respectively — high enough to penetrate the stratosphere. My hunch proved correct!
Air molecules, along with the tiniest dust particles and some aerosols, scatter blue and violet light from the Sun's white light to color the sky blue. Violet is scattered the most, and if our eyes were as sensitive to this color as they are to blue, the daytime sky would lean toward violet. Scattering redirects the shortest wavelengths in sunlight into billions of slightly different directions, diffusing it to produce the familiar blue sky.
At sunrise and sunset, sunlight takes a much longer path through the atmosphere and passes through far more air than usual, up to 40 times the amount compared to its summer noon position. Further, that air is far denser because it's at the bottom of the atmosphere where both air and aerosol densities are greatest. More scattering results, with both blue and green removed from the solar spectrum. A now deeply-reddened Sun illuminates clouds, trees and faces in warm hues like a spotlight shining on a stage.
Volcanic dust high in the stratosphere, where the Sun shines for a short time after it has set for the troposphere (the lowest layer of the atmosphere), also scatters blue light. Mixed with the predominately red and orange light present in the lower atmosphere, the twin glows combine to create a diffuse purple veil in the western sky at sunset or in the east at sunrise.
On August 28th, I drove to an open field and watched the western sky starting at sunset. A pale, pinkish-orange blush about 30° high (three fists) appeared over the sunset point 10 minutes after sundown. It deepened to a vivid, rosy-purple glow about 30° across that reached greatest intensity 15 minutes after sunset. Closer to the horizon the sky glowed the color of ripe cantaloupe fruit. Straining my eyes, I could see faint striations nearer the horizon which I interpreted as translucent bands of volcanic dust.
The brevity of the purple light surprised me — 20–25 minutes after sunset it had all but disappeared. But the orange-pink light of tropospheric twilight, mingled with a hint of stratospheric aerosols, still lingered within 15° of the horizon. All told it was a half-hour of pure gorgeousness. I encourage you to look for the purple light over your neighborhood at the next sunset or sunrise. The sight is easy to capture on a mobile phone, and binoculars may show signs of ash layers. The phenomenon is widespread. Searching online in recent days I've seen photos and heard reports from Ohio, Kansas, Arizona, California, and Canada.
One of the longest lasting ash-aerosol events occurred in the wake of the Mount Pinatubo volcano which erupted on June 15, 1991. According to the Aerosol Research Branch of NASA's Langley Research Center, the eruption "increased aerosol optical depth in the stratosphere by a factor of 10 to 100 times normal levels measured prior to the eruption." Aerosol optical depth is a measure of how much light airborne particles prevent from passing through a column of atmosphere.
Twilight effects lasted more than two years, and the millions of tons of debris in the atmosphere filtered the light of celestial objects for a similar amount of time. I recall not being able to reach my normal limiting magnitude for years until the dust cleared. Fred Espenak, astrophysicist and eclipse chronicler, called the December 9, 1992, total lunar eclipse the darkest in a decade. Whether we'll experience a noticeable change in sky transparency this time around remains to be seen.
In nature, great power often creates scenes of great delicacy. Just as the terror of a supernova sculpted the tendrils of the Veil Nebula, so too these recent ruptures in Earth's crust have inspired jaw-dropping twilights.