WRAL WeatherCenter Blog

Science in the sky: Anatomy of a rainbow

Posted May 28

Rainbow in Apex: May 25, 2017

As summer unofficially begins this weekend (summer officially begins here in the northern hemisphere at the solstice, June 21) we’ve already begun seeing the staple of summer weather in our area: isolated thunderstorms in the afternoon and evening.

As those storms pass, we are often treated to rainbows.

That is just what happened Thursday evening when a series of small storms passed through the area.

Occasional sprinkles didn't impede preparations for Apex High School's year-ending pops concert at Koka Booth Amphitheater in Cary. Showtime was a different story. The combined choirs were barely into the opening song when a small but heavy storm put the show on pause.

Fifteen minutes later, the crowd was rewarded with one of the most brilliant rainbows I’ve seen.

You probably know that rainbows are produced by sunlight passing through a raindrop. The light is bent or refracted because the denser water causes the light to travel more slowly. That light, now separated into its component wavelengths (colors), is reflected off the back of the raindrop and back out producing a colorful arc across the sky.

Rainbows are actually circles, centered on a point directly opposite the sun. We see just the portion of that circle above the horizon though. Rainbows most often appear in the early morning and late afternoon. The lower the sun, the more rainbow we see. Look closely and you'll sometimes find much more though.

Rainbow photo: Annotated version

The large raindrops of that storm and quickly clearing western skies produced an intense rainbow with narrow, well-defined bands of color. Small raindrops produce wider bands of color which overlap recombining those colors to appear more white.

Sometimes a broader, fainter bow appears above the primary bow. This happens as light is reflected once more inside the raindrop. That additional reflection reverses the color order in the secondary bow. Secondary bows are 1.8 times as wide as the primary and less than half the brightness.

Faintly visible just below the primary bow is a supernumerary arc. These alternate pink and green and are the result of interference of light as it exits the water drop.

Light is also reflecting off raindrops. This causes a noticeable brightening of the sky inside the primary bow. Similarly, a noticeable darkening of the sky between the primary and secondary bows is caused as light is reflected away from our eyes. This area is known as Alexander’s Dark Band, named for Alexander of Aphrodisias, who first described the phenomenon in AD 200.

Several in the crowd insisted they saw a third dim bow above the secondary bow. They did not. They were looking in the wrong place. In 250 years, only five scientific reports of tertiary rainbows are known to exist.

While each bow is created through the same refractive and reflective process inside raindrops, third (tertiary) and even fourth (quaternary) bows are extremely rare. These form around the sun, not opposite the sun as primary and secondary rainbows do. These higher order rainbows are usually are hidden by the sun’s glare, conditions have to be just right to see them.

Raymond Lee, a professor of meteorology at the U.S. Naval Academy, and optics expert Philip Laven described the conditions needed to create higher order rainbows in their paper published in Applied Optics in 2011. The sun breaking through dark thunderclouds following a heavy downpour of nearly uniform sized raindrops is required.

The evening of music was topped off when, as if on cue, the International Space Station rose directly behind the stage and over the crowd during the combined orchestra and chorus finale.

Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on Twitter @rtphokie.


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