It was good to see the sun again after an entire week of rain during the middle of January. A low-pressure system had established itself over North Georgia, and the front that brought it refused to move out for several days.
The concept of air pressure can be confusing for nonscientists. We know that if you drive from Gainesville to Suches, you’ll find yourself in much thinner air because Suches is a full 1,600 feet higher up. So, shouldn’t the pressure be low there all the time, and Gainesville should have mostly sunny weather because of the higher air pressure?
Not so. There are two ways of looking at air pressure. The first is the observation that pressure decreases as the altitude increases. It explains the thinner air on mountaintops, or the way you run out of breath more quickly when climbing stairs at an overlook on the Blue Ridge Parkway.
But this isn’t what’s important in meteorology. When dealing with weather, we’re interested in the changes that occur between places. That’s why the weather maps that you see in the media make a couple of important assumptions: All the places on the map are at the same altitude, and everything is happening at the same time. Without that, we wouldn’t be able to tell when there’s a low (a storm) moving through our area.
The existing air pressure differences between Suches and Gainesville, caused by altitude, would muddle up the picture too much. But after applying a correction factor to the pressure readings, we can see how the two towns compare, and whether a low or a high might be moving from one to the other.
This is especially important when pressure changes are strong. If you have a barometer on the wall, and the needle drops rapidly (say, 5 to 10 millibars in a couple of hours), it means there’s a severe storm approaching. This isn’t accurate enough for the most violent items, twisters for example, but it can warn you ahead of a strong winter storm.
A fast rise of the needle on the barometer indicates clearing. That’s usually not a bad thing. In the winter, though, it can mean heavy wind and very cold conditions.
After a big frontal system has moved through and dumped rain or sleet, we often see two days of biting cold wind while the Canadian air is chasing after the low, and high pressure is settling into Hall County.
Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.