Rainbow Bridge
December 02, 2004
Provided by: Stu Witmer
Summary authors & editors: Domenico Licchelli; Jim Foster; Stu Witmer
While on vacation in British Columbia, Canada this past September, we encountered considerable rain but were treated to a number of rainbows. This pair of bows was among the best. Not only were these the "flattest" rainbows I've ever seen, but the secondary bow (upper bow) was nearly as vivid as the primary (lower bow). Note the reverse order of the colors in the upper bow compared to the lower one -- red on the inside means a second reflection of sunlight occurred in the raindrops.
The time of day (1:30 p.m. local time) and date (September 17, 2004) this photo was captured explains the flatness of these rainbows. Since the angle of minimum deviation of sunlight in raindrops is approximately 42 degrees, rainbows cannot be observed if the Sun is higher than 42 degrees above the horizon. Thus, rainbows are generally more often seen near the end of the day, in the late afternoon or early in the morning. However, as summer transitions into fall, the Sun is not nearly as high in the sky, especially at higher latitudes. Consequently, a rainbow will be seen very near the horizon, and appear rather flat, when the Sun is closest to the zenith or about 40 degrees above the horizon. Because this photo was taken near the time of the equinox and not far from solar noon, and because the latitude of Lake Okanagan is approximately 50 degrees (north latitude), the Sun was likely about 35 degrees above the horizon. See also the Earth Science Picture of the Day for July 11, 2003.
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