For half a year now, rainbows have been conspicuously absent from San Diego skies. This will surely change within a few weeks as we encounter the first of several winter storms spinning out of the Pacific Ocean. Rainbows thrive on two necessary ingredients: rain and sunshine. Therefore, you can expect to see -- or even seek out, if you're willing to travel around a bit -- a rainbow wherever sunlight is shining on raindrops.
Rainbows seem magical, yet their true nature has been recognized since the 17th Century. In a treatise published in the year 1637, René Descartes elucidated the optical mechanism by which sunlight can refract and internally reflect through a spherical water droplet so as to emerge in different directions. One internal reflection results in light that exits, for the most part, at about 42 degrees from the antisolar point (the direction opposite the sun). Two internal reflections result in light that concentrates at an angle of about 51 degrees from the antisolar point, creating a fainter, secondary bow outside the curve of the 42-degree primary bow.
Dispersion -- a separation of colors -- always occurs when white light is refracted. Sunlight passing through a rain droplet separates into narrow bands of color -- violet, blue, green, yellow, orange, and red. Within each color band, we're seeing light rays of a particular color following symmetrical paths through many rain droplets. Asymmetric light rays through rain droplets result in a diffuse, white glow that makes the area of the sky inside the curve of the primary bow appear brighter than the area just outside it.
Each rainbow is unique to you, the observer, because you see a unique set of raindrops sending particular colors of sunlight your way. With a little forethought and the ability to move around, it is possible to purposefully "create" your own rainbow in the sky. To chase and catch a rainbow, place yourself between the sun, in one part of the sky, and clouds shedding rain in the opposite part of the sky.
If the sun lies very near the horizon, look for the "red rainbow." The scattering of sunlight through the atmosphere removes nearly all colors but red in the sunlight and from the rainbow that that same light produces. Here's yet another rainbow oddity: when the bright full moon shines on rain clouds, there's the slim chance of seeing a "moonbow." This pale arc appears colorless in the sky -- but only because human night vision, which operates in low-light situations, is insensitive to color.
A full moon occurs this Saturday night, November 8 -- but let's hope that rain clouds do not gather on that evening. For the second time this calendar year, a total lunar eclipse graces the sky at dusk. As darkness falls (around 5:15 p.m.), it should be possible to spot the pale, reddish gray moon -- fully within Earth's shadow at that time -- slowly rising in the east. The reddish color on the moon's face is the result of sunlight passing through Earth's murky atmosphere and refracting into Earth's shadow. At 5:31 p.m. the moon will start to exit from the shadow. A series of partial-eclipse stages will ensue, resulting in the return of the round "full" moon by 7:04 p.m.
Early November usually brings clear early-evening skies to San Diego, except possibly for areas very near the coast. Inland, toward the mountains and desert, there's an excellent chance of cloud-free viewing. Seize the moment, and enjoy one of nature's predictable spectacles this Saturday night!