A Rendezvous with Rainbows

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A rainbow is one of those delicate miracles that teaches that a greater-than-human mind had a role in the Creation. And, regardless of our religious beliefs, most of us find it astonishing that simple raindrops, falling in the rays of the sun, can separate white light into colors . . . and suspend those vivid, glowing hues in a vast arc across the sky! What's more, some of the scientific aspects of rainbows—and of their relationship to the environment in which they take place—are equally fascinating, particularly since having a better understanding of the whys and wherefores of these fleeting bridges in the sky can help us to find and enjoy more of them. And there are an amazing number of different kinds of rainbows . . . which, together, make up a wealth of wonders to equal any imagined pot of gold.

In this journey through the realm of rainbows, we'll discover that many of the colorful arcs can appear in the sky at one time, and we'll learn what happens to them as the sun sets. We'll meet rainbows that lie on the ground and seem to stretch to infinity . . . rainbows that are as close as spiderwebs and lawn sprinklers . . . and others that can be seen a state away. There's a red rainbow . . . a white version (which sometimes shows or ange and blue) . . . a rainbow pillar . . . a "rose" of rainbows . . . and a rainbow for each person (and even each individual eye)!

WHERE TO LOOK

Rainbows seen in the western morning sky will usually mean that precipitation is headed your way. But the phenomena can be expected to appear most often on summer afternoons when a shower or thunderstorm—hastening away to the east—is suddenly illuminated by the sun. They're formed, you see, when light is refracted and reflected in raindrops, and—because the various wavelengths of visible light (colors) bend at different angles when passing through the droplets—the colors separate and spread out in a band that's about three times wider than the sun or moon (when these heavenly bodies are viewed from the earth). The result is an astonishing circular spectrum, which is cut into an arch by the horizon and which ranges from red on the top, through a series of hues, to violet on the bottom.

Normally, we think of a rainbow as being made up of seven different colors, but this isn't strictly true. The variety of hues will depend upon a particular bow's visibility and upon the color recognition of the observer.

The common, or "primary", rainbow curves around a place opposite to the sun's position in the sky (this is called the "antisolar point") at an angular distance of about 42° from that spot. And, since your fist—held at arm's length—is about 10° wide, 42° will equal approximately four widths of your fist. To locate the antisolar point, just stand with your back to the sun, and the point will be at your head's shadow. Then, holding your arm straight out in front of you, you can measure four fist-widths up from that shadow to find the most likely location of the bow.

You'll note, too, that the largest arcs are seen when the sun is setting, because—at that time—the antisolar point draws closer to the opposite horizon, and fully half of the rainbow's circle can be visible. On the other hand, when the sun is at an angle greater than 42°, the rainbow will be completely below the horizon. Therefore, these colorful arches can never-in the U.S. and much of Europe, at least—be glimpsed around noon on a summer day. However—if the conditions are just right—as the sun descends a bit, you may be lucky enough to actually see the top of a rainbow begin to appear on the eastern horizon . . . swelling into beauty like a sunrise.

Another strange sight sometimes occurs when you view a rainbow at sunset. As old Sol drops lower in the sky, the arch's colors will fade, one after another, until only the red band remains. This scarlet rainbow can even be seen for a while after the sun goes down, because some of the raindrops are far above ground level and are thus still illuminated. Finally, however, the ends of the red band will slowly disappear . . . until only the peak of the bow is left as a shrinking, glowing red spot in the darkening east.

The only way to see the bottom curve of a rainbow (or perhaps the full 360 ° circle of it) is to be in a position that places a great many rain droplets between you and the ground . . . as might be the case when looking down on a shower from the vantage point of an airplane. Don't be fooled, however, if you notice several small, concentric circles of vivid color floating on the clouds below your window. They represent another wondrous (but far more common) sky phenomenon called "the glory". And, since only red and green blue are visible in that strange sight, it can't be classed as a full-circle rainbow.

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