Sun Dog
Published on Jul 27, 2006 at 8:37 pm.
1 Comment.
Filed under skywatching.
Last night driving home after my evening class, I saw a very pretty sundog. Sundogs, or parhelia, are atmospheric phenomina. The best time to see them is when the Sun is not too high in the sky, and the sunlight is shining through cirrus clouds. Cirrus clouds are those very high, thin, wispy ones. They form in the upper parts of the troposphere, the layer of Earth’s atmosphere where weather occurs. Forming at such high altitudes, the cirrus clouds are composed of ice crystals, not water droplets. There are a couple types of ice crystals that commonly form. Some are long needlelike structures, and others are flat hexagonal plates. The flat ones tend to float around horizontally. So, when light enters the side of one of them at a shallow angle, it can bounce around in them and exit another side. If you happen to be looking in that direction, then you see this refracted light. Different colors of light bend differently in the ice crystal, just as they do in water droplets, so you see different colors from different ice crystals, the particular color being determined by the relative orientation of the Sun, the ice crystals, and you. This looks sort of like a piece of a rainbow. You can see a picture of a sundog here.
In addition to the flat ice crystals, there are the longer ones that I mentioned. They, too, can catch light and refract it to you. They tend to float with their long axes vertical, so you get the right orientation from all directions around the Sun, at a distance of about 22 degrees from the Sun. A picture of this 22 degree halo can be found here. The sundogs are also 22 degrees from the Sun, so they sometimes appear as bright patches to the left or right of the Sun. That can be seen in this photo.
These phenomina are really not all that uncommon, but a lot of people have never seen them for some reason. My guess is that they simply didn’t notice them. The 22 degree halo can be seen with the Sun somewhat higher in the sky, but the Sun needs to be reasonably low in order to see the sundogs (notice how low in the photos). That can happen for several hours after sunrise, or for several hours before sunset. In the winter, if you are far enough from the equator, the Sun appears pretty low in the sky all day, so you get more opportunities to see these things. And, even thought it takes ice crystals for them to be visible, it does not have to be cold where you are. It was in the mid 90’s here yesterday, and I saw the sundogs. That is becuase, while it is hot on the surface, the temperature drops as you cllimb in altitude. It does not matter how hot it is on the ground, if you get high enough, you will see sundogs. Now, if it is REALLY cold on the ground, like winter very far north or south, then the surface temperature will be well below 0 degrees, and you might get some ice crystals at lower altitudes.
But, the next time that the Sun is low in the sky, and there are cirrus clouds in the area, go look to see if you see any sundogs.
-Astroprof
Astroprof’s Page » Parhelia on November 23, 2007 at 10:20 pm: 1
[…] About a week or so ago, I was driving home and I looked up towards the Sun. It was late in the afternoon, and the Sun was fairly low in the sky. Off to either side of the Sun was a small rainbow pattern in the sky. They were parhelia (often called sun dogs). I first wrote about sun dogs over a year ago, but this time I have some of my own photos, so I thought that I’d write about them again. The word parhelia means “with the Sun,” and it is the scientific term for the phenomenon, but I rather like the layman’s term sun dogs, since that is how I learned them. I managed to get home in time to grab my camera and take some pictures. […]