Rainbow season has now ended. This magical phenom is rare in winter, since it requires a sky with separate clouds instead of the overcast we commonly get from November through April.
Instead, these days we sometimes see even more vivid rainbow-like phenomena that go entirely unrecognized. Or misidentified. A few years ago, a well-known diffraction effect caused by sunlight striking low cirrus clouds produced a vivid horizontal band of brilliant colors. The newspaper that published it captioned it as being a “fire rainbow.” But there’s no such thing.
The same story applies to cloud iridescence, the vivid colors often seen on the fringes of white clouds. “Look, it’s a rainbow!” people excitedly exclaim. So let’s go back to junior high Earth Science for a minute.
A rainbow happens opposite the Sun and nowhere else. So if what you’re seeing isn’t exactly opposite the Sun, it’s not a rainbow. It appears when sunlight strikes falling rain. So if it’s 20 degrees outside so there’s no way rain is falling, you can forget about rainbows too.
Whenever you’re positioned between lowish sun and rain, there will probably be a rainbow. It’s large, invariable, 42-degree radius, always centered on the shadow of your head, dictates its limitations. Between April and September, rainbows can’t be seen between 10:30 a.m. and 3:30 p.m. because the sun is then higher than 42 degrees and the anti-solar point lurks too far below the opposite horizon. It makes sense that rainbows display their largest arcs when the sun is lowest.
Blue is always on the inside, red on the outside, but the intensity varies. Often, nine degrees outside the main rainbow a second, weaker one appears, with colors always reversed. All rainbows are caused by reflection and refraction within water droplets, and this secondary bow is fainter because the light has undergone an extra bounce within each drop.
The sky between the two rainbows is darker than anywhere else. The name for this phenomenon, Alexander’s Dark Band, is oddly similar to the old song with “ragtime” in the middle. It honors Alexander of Aphrodisias, who in 200 AD first noted the phenomenon. Point it out the next time you observe it and see if possessing such arcane knowledge, along with the Aphrodisias part, has any salutary effect on your date.
A lawn sprinkler produces a nearby rainbow while rain-cloud rainbows can be miles away. Yet they’re all the same apparent size. It’s also true that your rainbow is unique. Someone next to you receives light from an entirely different set of raindrops. They see a separate rainbow.
My favorite fact: You can never see a rainbow and its reflection. That’s because rainbows are not real 3-D objects. Nor can rainbows cast shadows. Their fairyland reputation is well deserved. Everything about rainbows is either strange or beautiful or both.
Since most people call any spectral colors in the sky a rainbow, what they’re usually seeing — especially during these cold months — is a totally different event. Nature lovers habitually look for these. The three most colorful ones are sundogs, the 22° halo, and the CZA. All are caused by refraction (light bending) within hexagonal ice crystals, which is what high thin clouds are composed of, even in summer.
Whenever the Sun shines through high thin clouds, block out its brilliance with an outstretched hand, and gaze far around it. Do this and I guarantee you’ll see the beautiful 22° halo once or twice a week. It’s always the same size, and with red on the inside. A spread-apart thumb tip to pinky held at arm’s length accurately measures the gap between the Sun and this halo.
When the Sun is about a third of the way up the sky, look very far above it for the CZA, the circumzenithal arc. This is visible only a few times a year, but is worth hunting for since its gorgeous colors are so deep and saturated, they make a rainbow seem pale by comparison. It appears against a blue or slightly milky sky and is always very nearly directly overhead — a location an actual rainbow never occupies.
Finally, when the Sun is fairly low and feathery cirrus clouds are bountiful, look directly to its right and left for sundogs. Technically called parhelia, these brilliant color patches are sometimes plain white but always breathtaking even if they’re common. We get them once or twice a week here.
Sundogs, haloes and the CZA. None are rainbows but each is a pot of gold.