Parhelia
Published on Nov 23, 2007 at 10:20 pm.
2 Comments.
Filed under atmosphere, skywatching.
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.
The sun dogs are actually part of a larger structure called the 22° halo (or sometimes simply called a 22° circle). The 22° halo (and sun dogs) are caused by light passing through small hexagonal ice crystals suspended in the atmosphere. Frequently this is in the form of high altitude cirrus clouds, but it can happen at lower altitudes in extremely cold weather when there is an ice fog. When the ice crystals are randomly distributed you get a circle around the Sun. This circle is called the 22° halo because it is about 22 degrees from the Sun (a bit less for red light, and a bit more for blue light). But, often the ice crystals are shaped like flat platelets, and if they are oriented horizontally, then they form the sun dogs. But, since the ice crystals are rather flat platelets, the Sun needs to enter the crystal at a fairly shallow angle with respect to the plane of the crystal in order to exit the other side to form the sun dog. That means that the Sun must be at a fairly low altitude in the sky in order for an observer to see the sun dogs.
But, sun dogs and the 22° halo are not the only atmospheric phenomenon that you can see with cirrus clouds. In the following photo, you can see another ice crystal image: an upper tangent arc. If you look carefully at the photograph, you can see sun dogs (with one being more prominent than the other) and a faint 22 degree halo. And at the top of the halo, you can clearly see an inverted arc. That is the upper tangent arc. The upper tangent arc is also an ice crystal arc, but with longer hexagonal ice crystals rather then the flat ones.
These features are cloud phenomena, so you might wonder what interest they hold for astronomers. Well, astronomers are sky watchers, and there are clouds in the sky. We tend to have to learn a bit of meteorology. After all, clouds interfere with what we are looking at. I have spent an awful lot of my life looking up at clouds. But, these thin cirrus clouds are interesting because of this particular optical phenomenon. So, the next time that you see cirrus clouds with the Sun low in the sky, keep an eye out for parhelia.
-Astroprof
(Images by Astroprof)
Astroprof’s Page » Unidentified Flying Sundog on January 18, 2008 at 11:35 am: 1
[…] Two days ago, I wrote about an UFO sighting over Stephenville, TX, that has been getting a lot of press attention. Well, the local news paper, the Fort Worth Star Telegram, has published a photograph purportedly of the UFO. It is an excellent photograph of a parhelion! Here is the photograph, courtesy of Sean Kiel, a truck driver who took it with his cell phone: […]
Jo Annie Chapman on April 21, 2008 at 7:35 pm: 2
Sunday, 20 April 2008, a 22 degree parhelia was quite evident in the southern sky here in South Central Missouri. I noticed it early afternoon and it remained visible for a few hours. It was a magnificent sight to behold - a perfect circle of “rainbow” around the sun. High cirrus clouds misted over the sun most of the afternoon.
Did anyone else see it?