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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

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).

Figure 3

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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