Hurricane season has been quiet so far. Other than a harmless tropical depression headed toward Iceland, there was nothing significant on the radar last week.
Like all storms, “depressions” are areas of low air pressure. So are hurricanes and continental storms. The topic is confusing in some ways. Everybody knows that the air gets thinner with altitude, so the pressure on top of Brasstown Bald is lower than in Gainesville. The walk path up to the tower at Brasstown Bald is 3,500 feet higher than Downtown Square. But this has nothing to do with storm formation. Weather charts that show highs and lows ignore elevation. This makes it possible to follow an area of low pressure across the county, and to map out the highs as well. In other words, every place on a weather map is treated as if it were at the same elevation.
Now we can get an idea of what those lows really are. In North Georgia, the heavy rains we’ve seen in the past two months have been mostly from coastal storms forming near the Texas Coast. The sea water temperature near Galveston is 84 degrees currently. This puts a lot of heat and moisture into the air. As the air drifts toward the land, it rises and a storm forms. The rising air has low pressure because air is being removed from the surface as it travels toward higher altitude. Soon, a thick bundle of rainstorms gets pushed into Georgia. The driving force is upper-air windflow, also known as circumpolar vortex. TV anchors tend to bring that mechanism to our attention only in the winter, when it sounds more threatening.
The process intensifies when high pressure develops over western Canada and Alaska. Nighttime low temperatures are still in the 50s at locations like Juneau, Anchorage and Fairbanks. Over cool, dry surfaces air gets heavy. It sinks down and spreads toward the rest of our continent. Upon encountering a large Southern rainstorm, it intensifies the activity by lifting the lightweight, moist Gulf air higher, causing even more rain.
This is nice in terms of keeping North Georgia green and pretty. But it also brings the potential for flooding. Urbanized areas are especially vulnerable because so much ground is impenetrable to rain. With more rainstorms coming, it’s wise to stay alert for flash flood warnings.
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.