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Tuesday, April 29, 2014

Chasing Rainbows

How to predict where a rainbow will be and how to set up your DSLR to photograph it to create dramatic rainbow-scapes

This Article Features Photo Zoom

A lot of photographers have images of beautiful, colorful rainbows. Unfortunately, many of them feature a Walmart parking lot in the foreground because the photographer just happened to see the rainbow and snapped a photo. It's possible to predict where rainbows will happen so you can previsualize and be in position to get a truly special image. Above: Rainbow over the Dyke, Kebler Pass area, near Crested Butte, Colorado.

The weather in Arches National Park that afternoon was stormy. Violent but brief squalls alternated with bursts of brilliant sunshine. It was "rainbow weather"—a pattern where rainbows were likely. But where would a rainbow appear? Hunkered down in my truck, I pulled out a topographic map, a printout of sun positions during the day and a chart I had created, which showed where a rainbow would intersect a level horizon for different sun altitudes. The place to go, I decided, was the Garden of Eden overlook. If a rainbow appeared, it would arc up and over the La Sal Mountains and the towers of the Windows area.

Rainbow over Uncle Bud's Hut, 10th Mountain Division Hut Association, near Leadville, Colorado. This image details the angle of polarization.
I drove to the overlook and waited. Dense clouds shrouded the sun. The moment of sunset arrived. I couldn't see the western horizon, but the situation looked hopeless. At the last possible moment, the sun must have found a gap between the clouds and the horizon. Red sunset light illuminated the sandstone towers. Suddenly, a rainbow appeared, precisely where I had predicted it would. I could only grab a few frames before the rainbow vanished.

To chase rainbows successfully, you need to know how to predict where they will appear and how to shoot them once they do. Many of the most beautiful rainbow photographs were shot with a polarizing filter, but beware: Polarizing filters can enhance rainbows, but also destroy them.

Rainbow Basics
A rainbow forms when direct sunlight strikes a curtain of raindrops. The primary bow appears when rays of light bend due to refraction as they enter the drop, bounce once off the back of the drop and bend again as they reemerge from the drop. Although light can escape from the drop at many angles, the laws of refraction and reflection dictate that a concentrated bundle of rays will bounce back at an angle of 42º to the incoming sunlight, as shown in Figure 1.

As Figure 2 shows, this bend-bounce-and-bend-again geometry dictates that rainbows appear in a circle, centered on the antisolar point, with an angular radius of 42º. Let's take that statement apart and make sense of it.

The antisolar point is the point directly opposite the sun. Imagine turning your back to the sun and looking at the shadow of your head. You're looking toward the antisolar point.

Rainbows have no defined size in feet or miles, since the curtain of falling drops may be 10 feet or 10 miles away. But they do have an angular size, which is 42º for the primary bow and 51º for the secondary bow. In other words, the primary bow appears when you look away from the antisolar point at an angle of 42º.

Figure 1

Figure 2

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