Georgia’s mountains, with the tallest peak reaching 4,784 feet above sea level, are well below the monster heights of the Western USA. In Alaska, Denali Mountain rises 20,310 feet above the Pacific Ocean. The Cascades in Washington State and the Sierra Nevada follow with an impressive 14,000 feet range. These kinds of heights approach the bottom of the jet stream, so they have a significant influence on the behavior of weather systems. It’s called the orographic effect.
Tall mountain ranges have a windward side and a leeward side. In North America, predominant wind direction is from the west and northwest. Seattle, Washington is located on the western side, the windward side, of the Cascades. Air from the Pacific Ocean rises on the slopes, cools and becomes more humid. When it exceeds its moisture-holding capacity, rain develops. This happens with such frequency that annual precipitation totals can be as high as 150 inches. Compared to that, Gainesville’s 54 inches and 107 rainy days per year look sparse.
On the leeward side of the mountains, air descends down the slopes and gets warmer. This lowers its relative humidity. At the bottom end of the mountain range, the air is very dry. The effect can be extreme, for example in Kennewick, Washington and surroundings, where average total precipitation per year is only 8 inches. By comparison, Las Cruces in the New Mexico desert receives 10 inches a year.
The effect of increased precipitation on the windward side, and dryer conditions on the leeward side, is present in North Georgia as well. But it’s less pronounced than in Western States because of our lower altitude. Blairsville and Hiawassee are located near the northwestern-facing slopes of the Blue Ridge Mountains. Wind from northwest to southeast produces a rapid increase in rainfall totals as it moves uphill. Blairsville, at 1,900 feet elevation, gets 57 inches of rain per year. 15 miles away and 1,500 feet higher, precipitation in that part of the Blue Ridge is in the 80-inch-plus category, according to the map from the National Weather Service. On the leeward side, rain totals decrease in a much gentler fashion.
In a more anecdotal setting, I’ve often observed that when wind is pushing clouds against the slopes of 700-foot-tall Banks Ridge in Habersham and Banks County, rain always falls first in Lula, Alto and Baldwin. These three towns sit squarely on the Eastern Continental Divide.
Rudi Kiefer, Ph.D., is a professor emeritus of physical science at Brenau University. His column appears Sundays and at gainesvilletimes.com.