Last weekend brought a number of clues that North Georgia’s weather is now shifting into its winter pattern. Saturday started out with clear sky. Around noon, clouds began to appear. They looked wispy, feather-like, coming from the south. Clearly, this was a warm front. The clouds were the cirrus type. They have a brushstroke appearance because they are the highest clouds that can form in the atmosphere. On average, they may be some 7 to 8 miles above the earth’s surface. It’s very cold up there, so no liquid water exists at that altitude. It’s ice crystals that give cirrus clouds their unique appearance.
When they start to fill the sky from the south, coming from a warmer region, cirrus clouds are the first messengers that a warm air mass is pushing towards our location. The earth is curved, so we first see whatever is tallest when it approaches. One can compare it to a ship’s mast that seems to be emerging from the ocean before we see the rest of an approaching ship.
Once the cirrus clouds have passed through, the warm front typically brings mild weather and a loose blanket of lower clouds. That happened on Sunday. The sky showed a great mixture of shapes and sizes. Weather watchers would call them cumulus and altocumulus, where “alto” denotes the higher-altitude variety of the knobby cumulus clouds.
The warm front is just one of three elements making up a typical U.S. winter storm. The greatest activity happens inside a low-pressure center, where the warm front joins a cold front. In combination, one can envision it like a pair of windshield wipers suspended from a common pivot. The first “wiper” is the warm front, with the Low (the “pivot”) well to the north of Georgia. The mild, humid air brought by the warm front can last a few days. Then comes the second wiper: the cold front. It’s the edge of a dry, cold, heavy mass of air coming from Canada. It did not make it to North Georgia last weekend because the Canadian air, called “continental Polar” or “cP”, hasn’t acquired enough punch yet to push into Hall County.
That will change. As winter approaches, we’ll see those frontal systems reach increasingly far south. Cold, dry, clear mornings will be a sign that a cold front has passed through.
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.