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Four rainbows in less than fourteen hours

The first rainbow, appearing around 5:40 p.m. E. D. T. on Jul. 24, 2014, was low in the sky.
The first rainbow, appearing around 5:40 p.m. E. D. T. on Jul. 24, 2014, was low in the sky.
H. Michael Mogil

Sometimes, if one is in the right place, at the right time, he/she will get to see a rainbow. If things work out in a correct meteorological manner, one might even see a double bow. The odds are very small when it comes to seeing three distinct rainbows within the same day. In Naples, FL, on Jul. 24, 2014, some people were treated to a rainbow troika.

As if three weren’t enough, at 7:00 a.m. on Jul. 25, 2014, another rainbow graced the Naples skies, this time almost overhead. That’s four distinct bows in less than 14 hours. It might not be a record, but it’s probably close.

Note that all of these bows occurred with different shower or thunderstorm clouds and all were evident within a 2-mile radius from a single location in North Naples, FL.

For each of these rainbows, the meteorological setting had to be “just right.” This required that rain be falling on the side of the sky away from the sun. It also required that clouds allow at least some of the sun’s rays to reach the opposite side of the sky. Spotty showers and thunderstorms ensured that both conditions were met.

Also, because the angle between the sun’s rays, the back of the falling raindrops and the observer has to be at least 48 degrees, the sun has to be around or less than halfway between its zenith and the horizon. Thus, except in special cases (e.g., on a mountain, in an airplane or in taller building), one doesn’t get to see a natural rainbow between about 9:00 a.m. and 3:00 p.m. Of course, one could create a rainbow using the spray of a garden hose or large enough fountain and simply control the angle of view to meet the above-noted requirements.

Rainbows form when light enters spherical raindrops, is bent (refracted) upon crossing into a different medium, reflects off the back inside surface of the raindrop, exits the raindrop (being refracted again) and finally reaches the eyes of the observer. The refraction process allows the “white” light we see from the sun to be broken up into its component colors. Red, being bent the least, appears on the outside of the primary rainbow.

This refraction effect is similar to what is seen in some “sparkling” gems, ones that seem to contain rainbow colors. Here, the bending and reflection of light is determined by the faces of the gem, faces often cut to maximize the sparkle and color. However, since one is looking at a single gem and not millions of raindrops, there is no bow.

On July 24, around 5:30 p.m., the first rainbow was evident to the east. With the sun still relatively high in the sky, the bow (Fig. 1) arced low across the sky. The second bow (Fig. 2) was much higher in sky (around 6:50 p.m.). This bow even had a faint double bow above it (Fig. 3). It’s hard to see, but the colors of the second bow (due a second round of internal raindrop reflections) are flipped. Purple is on the outside and red on the inside. In this case, lack of clouds above the bow was not a deterent. The clouds were in the process of raining themselves out. Even as the cloud disappeared, rain was still falling (Fig. 4)

At around 7:20 p.m., with the sun even lower in the sky, a third bow, against a lone, towering, distant cumulus cloud, showcases precipitation falling from the cloud (Fig. 5). This bow was much more vertical than the two earlier rainbows.

On the morning of Jul. 25, 2014, with the sun near the eastern horizon, the rainbow (just a small piece because only a small region of falling precipitation was evident) can be seen high in sky, almost overhead (Fig. 6). The precipitation from this shower probably didn’t reach the ground (and would be referred to as, “virga”).

Fig. 7 shows all four bows together to better highlight how the time of day and sun angle affected the position and altitude of each rainbow.

With more showery weather in store for the area, maybe the weather forecast should be rewritten as follows, “partly cloudy with a 30 percent chance of showers, but an 80 percent chance of rainbows, especially early and late in the day…”

© 2014 H. Michael Mogil

H. Michael Mogil is a Certified Consulting Meteorologist and self-professed math maven. He and his wife operate a mathematically-focused tutoring center in Naples, FL.

For more detailed information about rainbow formation, please check out these web links.

http://www.scribd.com/doc/18967370/42-Rainbow-Scientific-Explanation

http://www2.hesston.edu/Physics/Rainbows/research2_paper.htm