On August 27, 2019, I got up at 2:15 a.m., drove an hour and a quarter to the Long Lake trailhead in the Indian Peaks Wilderness, and hiked three miles to a location I last photographed in 1996 using a 4x5 field camera. My subject was a small stream that flows into Lake Isabelle. I planned to use the stream as foreground and shoot looking east as the sun rose over the lake. I was eager to learn the best way to use a digital camera to handle a difficult exposure challenge: shooting a constantly moving, reflective subject (the stream) lit by moment-of-sunrise light, with the sun in the frame. I had used a three-stop graduated neutral-density filter during my 4x5 shoot. This time I planned to shoot a heavily bracketed set of images and use Lightroom and Photoshop to produce the image I had in mind.
To my surprise, the best image of the morning proved to be the one I shot about 20 minutes before sunrise, when a strong golden glow developed along the eastern horizon. As I processed the image, I noticed that the sky above the golden band had an unusual purplish cast. The purplish color was even more pronounced in the water. Then I read Dr. Tony Phillips’ article in that day’s edition of the Spaceweather.com newsletter and the light dawned.
According to Dr. Phillips, about 60 volcanoes erupt every year, worldwide. Only a few eruptions, however, are powerful enough to pump plumes of sulfur dioxide gas into the stratosphere, where a series of chemical reactions eventually create tiny droplets of sulfuric acid and water. These droplets, called aerosols, can then spread around the globe. Two powerful eruptions occurred recently. Raikoke Volcano, in the Kirul Islands off the southern tip of the Kamchatka Peninsula, in far eastern Russia, erupted on June 22nd. Ulawun Volcano in Papua New Guinea, off the northeastern coast of Australia, erupted on August 3rd.
Newly formed aerosols reinforce the permanent aerosol layer in the stratosphere that I discussed in The Science of Glow Light. The greater aerosol density in turn enhances the twilight colors that can produce clear-sky glow shots. They can also have another interesting effect: adding a purple tint to regions of the sky that would normally be blue. We see purple when a combination of blue and red light strikes our eyes. To understand how such a combination happens, you need to know a bit about atmospheric optics.
Light as it comes from the sun contains all wavelengths. As sunlight passes through the atmosphere, it interacts with air molecules. Short wavelengths – blue light – scatter much more strongly than long wavelengths – red light. On a clear day, therefore, the sky away from the sun looks blue because blue light scattered out of the beam of direct sunlight reaches our eyes as we look in that direction.
At sunrise and sunset the light from the sun takes a much longer path through the atmosphere than it does at midday. The longer path length means more blue light scatters out of the beam, leaving the red light to travel straight through, without scattering. On a clear day at sunrise and sunset, the direct red light from the sun paints clouds, mountains, and desert towers with rosy hues, while the sky well above the rising or setting sun still appears blue.
Figure 2. Light from the sun that skims the Earth at the tangent point travels back up through the atmosphere and scatters off the aerosol layer, creating a golden glow near the horizon. Light from the sun that strikes the upper part of the atmosphere takes a shorter path length, so less blue light scatters out of the beam. Rayleigh scattering and scatter from the aerosol layer combine to add blue light to scattered red light, causing a purple glow to occur 15 to 20 degrees above the horizon about 15 to 25 minutes after sunset.
With that as preamble, you can now understand the purple glow. The purple glow appears 15 to 20 degrees above the horizon some 15 to 25 minutes before sunrise or after sunset. At that time, light from the sun, which is well below the horizon, takes a long path through the atmosphere, grazes the Earth’s surface at the tangent point, then continues back upward through the atmosphere until it hits the aerosol layer. During that long journey, most of the blue light scatters out of the beam. Many of the particles in the aerosol layer are larger than air molecules, which means they cause forward scattering of all wavelengths. The beam of red light scatters off the aerosol layer in the stratosphere and eventually reaches our eyes. Near the horizon, the reddish light is relatively pure (not mixed with other colors of light), producing a strong golden glow. That effect is stronger when the aerosol layer has been reinforced by the addition of aerosols from powerful recent volcanic eruptions. The sunlight striking the upper part of the atmosphere directly, without first passing through the lower troposphere, has taken a shorter path through less dense air, so less blue light has scattered out of the beam. Some of the red light in this beam is absorbed by ozone in the stratosphere. The now-bluish light scatters off the aerosol layer and comes down to our eyes. Additional blue light comes from the region of the troposphere that is in the shadow of the Earth. The bluish light then mingles with the reddish light that is scattered by the aerosol layer, as shown in figure 2. We see the combination of those two colors as purple.
Here's another way to look at it. On a clear day, when we look well away from the sun, the sky is always some shade of blue. To see purple, we just need to add a bit of red light to the blue light we're already seeing. We don't need to add more blue light. When the geometry is right and the air is clear at the tangent point, the red light caused by Rayleigh scattering bounces off the aerosol layer in the stratosphere, mixes with the background blue light, and gives us the purple glow.
According to Dr. Phillips, the aerosols created by Raikoke and Ulawun are not distributed evenly around the globe, although research by scientists at the University of Colorado showed that enhanced concentrations of volcanic aerosols were indeed present in the stratosphere over nearby Wyoming at the time I made my image. On some days, the twilight sky will exhibit a strong golden band near the horizon, with a purple glow above; on other days, not. A lot depends on conditions at the tangent point, the point on Earth’s surface where the light from the rising or setting sun skims the Earth’s surface on its long journey through the atmosphere. The tangent point when the golden and purple glows are peaking is always hundreds of miles away, well below your horizon. Dense clouds there can block the light that produces these atmospheric effects. You’ll never know until you go, however, so set that alarm for some absurdly early hour or prepare yourself to eat dinner late and get out there and shoot!
I shot this image on 4x5 film in 1994, not long after I purchased my first 4x5 field camera. While I certainly don't regard this image as one of my best, it clearly shows an intense purple glow in the sky, clouds, and water.
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