Ideas & Videos
Sundogs at Home
Sundogs in Ankeny, Iowa 1/3/18
Did you see the sundogs yesterday? So many of you posted amazing photos of their beauty. The scientific name for sundogs is parhelion. This comes from Greek “par” meaning beside and “helia” meaning sun. Parhelion are literally beside the sun. They got the nickname “sundog” because the follow closely behind the sun just as a dog would follow its owner. In other parts of the world, sundogs are called mock or phantom suns. Occasionally, sundogs will form into a full halo around the sun. This happens most frequently in the polar regions of Earth.
The sundogs we saw yesterday were caused when the cold temperatures froze the moisture, or humidity, in the air forming ice crystals. The ice crystals are heavier than the surrounding air, so they begin to sink lower in the sky. This creates a steady stream of ice crystals in the sky for the sunlight to pass through. When the white light of the sun passes through the vertical line of ice crystals, it separates into the light spectrum we know as a rainbow and a sundog is born. The refraction, or separation, of the sunlight is why sundogs often appear to have a reddish tint on the side closest to the sun and a bluish/white tint on the side farthest from the sun, just like the colors in a rainbow you may see after a thunderstorm.
In most regions, sundogs occur more commonly during very cold weather. This is for two reasons. The sun is lower in the sky (on the horizon) during the months of January, April, August, and October. Sundogs appear at the same distance from the horizon as the sun and 22 degrees distance away from the sun. When the sun is lower in the sky, the sundogs are more visible to us. As the sun rises higher into the sky, the line of sight changes and the sundogs rise out of our sight. Sundogs also occur more commonly during cold weather because the moisture in the air is more likely to freeze to form the necessary crystals that form sundogs. However, in warmer weather, high level clouds called cirrus clouds are the cause of sundogs. Because our atmosphere gets colder the higher you go, the moisture in high level cirrus clouds freezes to allow sundogs to form. Since cirrus clouds often form cirroform clouds which is a fairly accurate predictor of approaching warm weather and rain. Check the forecast! Will our sundogs help us predict the weather?
You can recreate a similar effect with a crystal from a chandelier, a prism, or crystal glassware with crisp edges. You can even use the crystals on your royal tiara to investigate science! (You can also order prism balls like the one pictured here on Amazon and other sites.) Have your child hold the glass in one hand and move a flashlight closer and further away from the crisp edge until a rainbow forms. In my case, I used the details on the stem of a champagne glass and the flashlight on my iPhone. Just like the sundog and sun, the rainbow should appear about at about a 22 degree angle from the line between the flash light and crystal edge. This change from the light of the flashlight to the rainbow is called refraction. It is the “bending” of white light to form the full spectrum of visible light: red, orange, yellow, green, blue, indigo, and violet (ROY G BIV). Refracted or bent light does not always produce a rainbow. It can also change the way we see items. This is because light travels at different speeds through substances like water compared to the air. Try add water to a glass until it is half full. Have your child add a longer object such as a pencil, popsicle stick, or wand. See what happens? What do you see?
