by Stan Tekiela
April 17, 2011
Photo by Stan Tekiela©
On a recent trip to Yellowstone to photograph wolves and foxes, the air temperature one morning hit 43 degrees below zero. Not the wind chill mind-you but the actual air temperature. Besides the obvious problems that comes along with such extreme temperatures, some fairly amazing and cool natural phenomena happen. For example about an hour after the sun came up on that clear morning, the largest and clearest sundog I have ever seen appeared.
Certain conditions need to be in place before a sundog can occur. First the sun needs to be low in the sky. Also, it needs to be cold, although it doesn't have to be extreme cold and in fact it can happen at any time of year. And the winds need to be light to set the sundog in motion. Oh, you also need clear skies and sunshine to make this work.
The air temperatures in the upper atmosphere at any time of year are very cold so any moisture found is in the form of ice crystals. Clouds, for example, are also made up of tiny ice crystals. The difference is the ice layer that forms the sundogs is very thin and the ice that forms the clouds is not. When the winds blow they move the thin layer of ice crystals around in the air lifting them higher into the atmosphere. Eventually the ice crystals start to fall back to earth. Each ice crystal is hexagonally shaped and lines up with other ice crystals during the decent. They fall like leaves falling from a tree. This is a very important part that creates the sundog because sunlight enters each ice crystal's thin edge and the sunlight is refracted (split into the spectrum of visible light) just like a prism. The more perfect the alignment of the falling crystals to the horizon the smaller the sundog. The more wide spread or less alignment of the ice crystals the more spread out or visible the sundog. Days with more wind spreads out the ice crystals and makes for a better sundog.
The sundogs appear on either or both sides of the sun and look like mini rainbows. They always appear in a position 22 degrees lateral (to the side) of the sun and parallel to the horizon. You can find 22 degree by holding up your outstretched arm with all your fingers spread wide. The distance from the tip of your thumb to the tip of your little finger is about 22 degrees. Place the tip of your thumb near the edge of the sun (caution, never look directly at the sun) and at the end of your smallest finger is where the sundogs appear. However sundogs are often so large and obvious that you won't need any help finding a sundog if it is out.
Over 2000 years ago the Greeks accurately noticed that sundogs around the sun were a good predictor of future precipitation. The reason behind this is ice crystals that form the sundogs where the beginning or leading edge of the clouds that would soon bring precipitation so they are good indicators or predictors of changing weather. I don't think this is particularly accurate during winter but I can see how it would make sense in summer.
The one problem with the sundog I photographed here is, the air temperatures was 40 degrees below zero. It made standing outside for any length of time to photograph this sundog a big challenge. Also challenging for the camera equipment and batteries. And it is never easy looking directly into the bright light to get a accurate assessment of the picture you are about to take. The things I do for the reading audience. Until next time...
Stan Tekiela is an author / naturalist and wildlife photographer who travels the US to study and photograph wildlife. He can be contacted via his web page at www.naturesmart.com
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