Anatomy of a rainbow ::

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.

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…

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