Look upward often to discover a variety of visual delights.
By Lowell and Kaye Christie, F47246
We spend a lot of time watching the sky. It helps us forecast weather conditions as we travel, exercises our imaginations with fanciful cloud formations, and provides a habitat for the soaring birds we like to observe. But add some distant raindrops or cirrus clouds full of tiny ice crystals, and you have the setting for some spectacular heavenly events.
Everyone pauses to look at rainbows during that brief time when sunshine and rain intermingle, but few realize how much is going on in the atmosphere the rest of the time. If you know where and when to look, you can see sun dogs, light pillars, brilliant halos, and colorful arcs in the sky. They’re all related to light reflected or refracted by moisture, either liquid or frozen.
We have a small glass sculpture on our kitchen windowsill that acts as a prism, breaking the morning sunlight into a rainbow of colors that paint themselves on the opposite wall. This happens because sunlight is actually a mixture of colors, each of a different frequency. When light enters a piece of glass (or a raindrop or an ice crystal) at an angle, the different color frequencies are deflected at various angles, splitting the light into its spectrum of colors. The result is a rainbow.
Halos and arcs that form in the sky usually have similar rainbow colors, because the basic process is the same. Other light displays, such as sun pillars, are caused by reflections of light off the flat surface of masses of ice crystals. These tend to assume the color characteristics of the light source, becoming redder near sunrise or sunset.
While rainbows are relatively infrequent, various types of halos are quite common, with certain forms occurring at least once or twice a week. But before we explain where to look, here’s an important warning: DON’T LOOK DIRECTLY AT THE SUN. Even with cloud cover, it still can be dangerous to your vision. Stand so that a building or tree blocks the actual image of the sun. Not only will it protect your eyes, it also will make it easier to see the halos.
We’ll start with the 22-degree halo, both because it occurs often and because it illustrates an important measurement for finding sky phenomena. This halo is caused by refractions of the sun’s rays through tiny hexagonal ice crystals in the sky between you and the sun. Even when the sky appears clear, there may be enough thin cloud cover to create halos, but visible clouds make the circle easier to see.
Fully extend your arm and spread your fingers wide. The distance between your thumb and the tip of your little finger will be approximately 20 degrees, just slightly smaller than the 22-degree halo you are looking for. Place your thumb over the spot where the sun would be (remember, it should be hidden behind a tree or building), and your little finger will point to the location of the halo if it’s visible today. Sometimes you will see only parts of the circle, depending upon where the ice crystals exist.
It is the shape of the ice crystals combined with the angle of the sun’s rays that creates this circular pattern around the sun, slightly darker on the inside of the circle (except for the location of the sun itself). Often the inner edge of the 22-degree halo will be tinged with red. You can sometimes see this same type of 22-degree halo at night around the moon. If it’s raining, look for moonbows, too.
Whether or not you see the 22-degree halo, now you know where to look for sun dogs. Sun dogs (also called parhelia) are bright spots in the sky 22 degrees to the right and left of the sun. They are easiest to see when the sun is quite low. Sometimes, when conditions are just right, the sun dogs are brilliant enough to look like two additional suns forming a horizontal line. They can occur with or without the 22-degree halo, and like many other sky events sometimes take on the colors of the rainbow, in this case with the red edge toward the sun.
If you are really lucky, you might see a combination of a complete 22-degree halo, a sun dog on each side, and a rainbow-like parhelic arc that matches the top of the halo. It makes it worth glancing toward the west in the late afternoons whenever you get the chance.
What some have called the most beautiful of the halos is the circumzenithal arc, found high in the sky when the sun is low. It looks like an upside-down rainbow, with its red edge toward the bottom. The red bottom will be pointing toward the sun, and it is more likely to appear when you can also see sun dogs.
Another arc formation is called the circumhorizontal arc, nicknamed the fire rainbow. It’s fairly rare, and unlike most of the other halo-related forms, occurs when the sun is very high in the sky. Look for this one around noon during the summer season. You can use a measurement similar to the one employed for the 22-degree halo, but you must use the width of two hand spans (about 44 degrees) below the sun, usually close to the horizon.
Anytime there are clouds in the late afternoon, we try to plan our evening to catch the sunset. Often we have to settle for a beautiful sunset (someone has to do it), but occasionally we get a complete light show. Called light pillars or sun pillars, these are strong columns of light extending upward (and sometime downward) from the sun. They are caused by light rays reflecting from the flattened surface of ice crystals, and normally extend five or 10 degrees (half a hand span) above the light source. (Yes, there are moon pillars, too.)
Often if there is an evening sun pillar, it will intensify after the sun actually drops below the horizon. This optical treat may last 30 minutes to an hour after sunset, and the pillar will slowly tilt northward as you watch. The column of light really remains vertical relative to the sun, but as the sun moves lower it is actually shifting slightly south, which accounts for the light beam appearing to move north.
Many other sky events are worth exploring, from the green flash that occurs as the sun sets, to diamond dust lighting up the sky in colder areas. Circular glories (another type of halo), coronas (rainbow rings smaller than the 22-degree halo), and many varieties of rainbows, double rainbows, and the often overlooked moonbow can be found. No wonder we spend so much time watching the sky. Maybe now you will, too.