Our Window on Nature

. . . exploring the world around us

Sundogs and Moonbows

Filed under: Sky — Lowell Christie -- July 7, 2015 @ 12:34 pm

LightPillar-YeahsooWe spend a lot of time watching the sky. It helps us forecast weather conditions as we travel, exercises our imagination with fanciful cloud formations, and provides 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 events in the outdoors.

Everyone pauses to look at rainbows as the air clears after a storm, 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 sundogs, 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.

On our kitchen windowsill we have a small glass sculpture 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 a different angle, 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 cause 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 can still 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 will also make it easier to see the halos.

We’ll start with the 22° 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 cirrus clouds 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° Halo you are looking for. Place your thumb over the spot where the sun would be (remember, it’s hidden behind a tree), and your little finger will point to the location of the halo if it’s visible today. Sometimes you will only see 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° circle will be tinged with red. During a full moon you can sometimes see this same type of 22° Halo at night. If there’s rain, also look for moonbows.

Whether or not you see the 22° Halo, now you know where to look for sundogs. Sundogs (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 sundogs are brilliant enough to look like two additional suns forming a horizontal line. They can occur with or without the 22° Halo, and like many other sky events sometimes take on the colors of the rainbow, in this case with the red edge towards the sun.

If you are really lucky you might see a combination of a complete 22° Halo, a sundog on each side, and a rainbow-like parhelic arc that matches the top of the halo. It makes it worth glancing towards 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 towards the bottom. The red bottom will be pointing towards the sun, and it is more likely to appear when you can also see sundogs.

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 used for the 22° Halo, but you must use the width of two hand spans (about 44 degrees) below the sun, usually close to the horizon.

Any time 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 upwards (and sometime downwards) from the sun. They are caused by light rays reflecting from the flattened surface of ice crystals, and normally extend five or ten 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. It may last thirty 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 seeming to move north.

There are lots of other sky events worth exploring, from the green flash 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° Halo), to the many varieties of rainbows, double rainbows, and the often overlooked moonbow. No wonder we spend so much time watching the sky. Maybe now you will too.



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