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|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.
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.
One practical conclusion from all this theory is that, in level country, rainbows can't appear unless the sun's altitude (its angle above the horizon) is less than 42º. At higher solar altitudes, the "rainbow" is below the horizon. In other words, don't bother chasing rainbows at noon. Realistically, you're most likely to see a photogenic rainbow when the sun is less than 20º or so above the horizon. In the mid-latitudes, that means rainbows are most often seen in the last (or first) two hours of the day.
If you can see the shadow of your head, you can quickly estimate where a rainbow will appear using a measuring tool built right into your body. Extend your arms straight out and touch thumb tip to thumb tip. Spread the fingers of both hands as wide as possible, and place the tip of your left little finger on the shadow of your head. Sight toward the tip of your right little finger and scribe an arc in the air, keeping the tip of your left little finger on the shadow of your head. That arc is where a rainbow will appear because, for most people, a double hand span covers an angle of roughly 42º.
Predict, Position And Shoot
Many of the best rainbow photographs show the rainbow arcing up and over something photogenic in its own right. You probably won't have time to move to such a location once a rainbow appears, so you should plan where you want to be in advance. To do that, you need to know where a rainbow will intersect a level horizon. First, determine the direction of the sun. If the sun is visible, measure its bearing with a compass. If it's hidden in clouds, you can consult an app on your smartphone. I like Sun Surveyor (iPhone and Android, www.sunsurveyor.com) and The Photographer's Ephemeris (iPhone and Android, www.photoephemeris.com). Or you can consult a printout of sun positions you made before your trip (Figure 3). I like the inexpensive Heavenly-Opportunity software package (Windows, ho.fossilcreeksoft.com).
However you get the sun's position, the next step is to subtract 180º from the sun's bearing. The result is the direction of the antisolar point, or to be more precise, the direction to the point on the horizon directly above the antisolar point (which usually will be below the horizon). Let's call this point the "horizon antisolar point."
If the sun is about to set, the problem is simple. The right limb of the primary bow will intersect the horizon 42º to the right of the horizon antisolar point. The left limb will intersect the horizon 42º left of the horizon antisolar point. This calculation is exact if the sun is right on the horizon and is still a reasonable approximation at any sun altitude less than 10º, or about an hour or less before sunset.
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Rainbow over the Windows area at sunset, Arches National Park, Utah.
Here's an example. It's sunset, and the sun is due west, which means its bearing is 270º. The horizon antisolar point is directly opposite the setting sun, or at 90º (270º - 180º = 90º). The right limb of the primary rainbow will intersect the horizon at 132º (90º + 42º = 132º). The left limb will intersect the horizon at 48º (90º - 42º = 48º).
If the sun is more than 10º above the horizon, the situation is slightly more complicated. Find the altitude of the sun in your Heavenly-Opportunity printout or smartphone app, and find the correct row in the table for that altitude (see above). Add the number in the second column to the bearing (direction) of the horizon antisolar point. The right limb of the rainbow will intersect the horizon there. Now subtract the number in the second column from the bearing of the horizon antisolar point. The left limb of the rainbow will intersect the horizon there. The third column shows the altitude of the highest point of the rainbow.
Here's an example. Assume, again, that the sun is due west, at a bearing of 270º and its altitude is 25º. Consulting the table, you can see that the rainbow will intersect the horizon 35º left and right of the horizon antisolar point, or at 125º and 65º, respectively. The top of the bow will be 17º above a level horizon.
Now that you can predict where a rainbow will appear, let's talk about how to shoot it. A complete semicircular rainbow that arcs high into the sky and stretches from horizon to horizon is a spectacular sight, but its appearance in a print is often underwhelming. That's because the band of colored light is only about 2º wide. You'll need a 20mm lens on a full-frame sensor to include all of the primary bow. You can easily end up with a very thin line of color curving across your frame and a whole lot of boring, gray sky underneath. The best rainbow shots are often taken with a moderate telephoto and include only a portion of the rainbow.
Using a telephoto gives you another advantage: You can use a polarizer to enhance the rainbow. Light from a rainbow is tangentially polarized, meaning the angle of polarization is along a line tangent to the rainbow. For a full, semicircular arc, the angle of polarization is vertical near the horizon and horizontal across the top of the bow. Light from the background is unpolarized, which means it can be thought of as a mixture of vertically and horizontally polarized light. Rotate your polarizer to allow the light from a section of the bow to pass through, and you'll darken the background by blocking that portion of the background light polarized at 90º to the rainbow light. The rainbow will stand out strongly against the darkened background. This only works with a telephoto, however, because you need to isolate a portion of the bow that has approximately the same angle of polarization along its length. If you include the entire bow in the frame by using a wide-angle lens, you'll find that rotating the polarizer will cause one portion of the bow to strengthen while another part disappears completely.
Armed with your new knowledge of how rainbows form and how to shoot them when they appear, you'll be well equipped to chase rainbows the next time a crack of thunder announces the arrival of rainbow weather. As the sun drops below an altitude of 42º, look for a gap in the clouds to the west, calculate where a rainbow will intersect the horizon, and position yourself so your hoped-for rainbow will complete an already strong composition. Never again will you be like the 60-something student who came up to me after my rainbow lecture and said, "All my life, I thought that rainbows appeared randomly." Now he knows better, and his photos will be better as a result.
You can see more of Glenn Randall's photography, sign up for his monthly newsletter, read his blog and learn about his upcoming workshops at www.glennrandall.com.