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The Science of Glow Light

Updated: Feb 14, 2021

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