The days are getting noticeably longer, but for North Georgia the coldest air is still to come. This has enjoyable aspects, too. Cold weather arrives in the form of fronts. Rain sets in, gets heavy sometimes, then clears out. Crispy cold temperatures tend to greet us the morning after that. Continental polar air, a gift from Canada, has arrived.
This polar air is very dry because it has acquired its characteristics over a cold land mass and not an ocean. Also, cold air can’t hold much moisture. It seems like a contradiction, but we to get our deepest-blue sky and brightest sunshine during those chilly days that follow a cold front.
It gets really interesting when cirrus clouds turn up in the sky because some other front is about to move in. Cirrus clouds are the highest in the atmosphere, where temperatures easily drop into the 40-below range. So they have no rain drops or any other kind of liquid water up there. Cirrus means that we’re looking at 100% ice crystals. Because it’s winter, the sun is fairly low and shines its light through those crystals at an angle, relative to us here on the ground. This turns every ice crystal into a prism. Look for wispy, feather-like clouds, high up in the sky, on an otherwise bright sunny day. Somewhere in there, on both sides of the sun, you should see sun dogs. They look like fragments of a rainbow and display all the colors of the light spectrum – red, green, blue and the transitions in between. Red is the one closest to the sun. That’s the longest wavelength, it’s the lazy one of the three and doesn’t get scattered as far out as the other colors. Blue is the most squirrelly. It bounces around easily, so it occupies the distant edge of the sun dog.
It’s a good idea to wear sunglasses for observing sun dogs. You don’t need the super-dark lenses that we use to watch an eclipse. But normal daytime sunlight contains enough invisible ultraviolet light to do damage to the eyes. I like orange-tinted sunglasses. On a color wheel, orange is the opposite of blue, so orange lenses filter out some of the blue wavelengths. This lets you enjoy a blue sky with very rich colors in nature’s prisms, about 8 miles overhead.
Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.