The Science of Glow Light

In my last blog post, The Most Colorful Light You’ll Ever See, I wrote about gap light, the extraordinarily colorful light that occurs when the sun finds a clear gap between dense clouds and the horizon. In this post I’ll explain an equally rare and beautiful but more subtle type of natural light I call glow light. My descriptions will focus on sunrise, but the same phenomena occur in reverse order at sunset.

To understand glow light, first consider what happens a few minutes before sunrise when the sky is completely clear. The sun is below the horizon, so no direct light can reach the land. The sky along the horizon where the sun is about to appear is usually white. In this situation, the dominant light source on the land is the blue sky directly above you. Photos taken during this so-called blue hour (really just a few blue minutes) have a pronounced bluish cast, giving the image a somber, cold feeling.

Arrive early enough, however, and you may see the land bathed in a warm, ethereal glow. In the most obvious scenario, a bank of clouds near the sun, which is still below the horizon, lights up pink or orange. Glowing clouds are beautiful, of course, and if they fit into your composition, by all means shoot them. But don’t ignore what those clouds are doing to the light on the land itself. If the bank of clouds is large enough, the warm light reflecting off the clouds can overpower the blue light from the sky and give the land a wonderful pink or magenta glow. It’s as if nature has suspended a giant, warm-toned softbox in the sky. Compare the color of the aspen trees in figure 1, taken 15 minutes before the almanac time of sunrise, when glowing clouds out of frame to the left were bouncing warm light onto the landscape, to the color of the light in figure 2, taken 17 minutes after sunrise, when the sun came over the ridge and illuminated the foreground aspen with direct light.

Figure 1: Twilight glow over Capitol Peak, taken about 15 minutes before the almanac time of sunrise.

Figure 2: Capitol Peak and aspen after the sun came over the ridge, about 17 minutes after sunrise.

A different glow scenario can occur even when the sky is completely clear. About 20 to 30 minutes before sunrise, if atmospheric conditions are right, the sky at the horizon to the east can develop a strong orange cast. On rare occasions, this light can be so strong it overpowers the blue light from the sky directly overhead, putting a diffuse warm glow over the landscape. As the sun gets closer to the horizon, the orange glow fades to yellow, then white. Photos taken a few minutes before sunrise have the familiar unappetizing blue cast because the dominant source of light on the land is the blue sky straight overhead. Finally the sun pops over the horizon. If the elevation of the horizon is the same or lower than your subject, it can be bathed in strong reddish or orange light.

Figure 3. A clear-sky glow shot of Dead Horse Point, Dead Horse Point State Park, Utah.

Figure 4. Dead Horse Point just before sunrise, when the light on the land comes predominately from the blue sky.

Figure 5. Sunrise at Dead Horse Point, Dead Horse Point State Park, Utah.

The conditions that produce a superb clear-sky glow shot are rare. For starters, the air at the tangent point – the point where the sun’s rays just graze the Earth – must be clear. Consider a sunset scenario. If you are in the middle of a flat plain, such as the wheat fields of Kansas, and the sun is just about to set, then you are standing at the tangent point. As the sun dips below the horizon, the tangent point moves west rapidly. The best twilight glows occur when the air at the tangent point some 20 to 30 minutes after sunset is very clear. By that time, the tangent point will be hundreds of miles to the west. Clear air causes Rayleigh scattering, in which blue light scatters out of the beam, while the red light travels straight ahead.

The second condition that must be met is that there is a layer of aerosols – tiny solid or liquid particles suspended in the air– high in the atmosphere that can bounce that warm light back down to Earth, where it can illuminate even the depths of a canyon. See figure 6.

Figure 6. Clear-sky glow shots require two conditions: clear air at the tangent point, where the sun's rays just skim the Earth, and an aerosol layer in the stratosphere that scatters the warm light from the sun back down to the observer and the surrounding land.

Predicting when such conditions will occur would require knowing both the extent of cloud cover and haze hundreds of miles away, below your horizon, and the characteristics of the aerosol layer in the stratosphere – a difficult if not impossible task for a landscape photographer. Fortunately, there’s an easy solution: arrive long enough before sunrise that you can set up and be ready to shoot 30 minutes before sunrise, and stay comparably late after sunset. You won’t always be rewarded, but the images you’ll produce when fortune smiles will make all the waiting worthwhile.

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