I took this picture Monday morning when I walked the dogs up to the top of the mountain. It was about 9:30, long after sunrise. Just right of center, where the steam plumes are, you can see the remains of the temperature inversion from the calm, clear atmosphere we had over Sunday night.
The foggy, linear stream on the left side of the plumes is the top of the inversion. If we had walked right after sunrise, the top of the inversion would probably have been more obvious, but it was already dissipating by this time. It would, however, have been at approximately the same level in the atmosphere.
An inversion serves as a cap on the atmosphere close to the surface. It traps moisture or pollutants that are beneath the top of the inversion. The temperature normally decreases as you go up higher in the troposphere. Air that is warmer than the air at the surface (like smoke from a brush fire) will tend to rise through the troposphere because it is lighter than its surroundings. In an inversion, the air actually gets warmer is you go up, so things like smoke will rise for a while, but will tend to stop at a low altitude. A very hot plume can push through the inversion and then continue to rise. The steam plumes are doing that.*
The steam plumes are coming from a paper mill. The two tall stacks to the right of the steam plumes are an old and a new stack at Plant Hammond, one of the two Georgia Power coal-fired power generating plants we can see from Lavender Mountain.
Plant Hammond’s active stack is 675 feet (205.8 m) tall. Although nowhere near the tallest stack in the world, it is tall enough to be on the Wikipedia list of the tallest stacks in the world.
It’s tall for a reason – the Clean Air Act, which goes back more than 50 years. That act has provisions that limit the concentration of pollutants at ground level. One might think that the logical way to do that would be to limit the emission of pollutants, but it happens that if you introduce the pollutants high enough in the air, they will have been diluted enough that by the time they can reach the ground, they will meet the standards. So the Georgia Power stacks are high enough to push emissions above the top of any reasonably probable inversion height. If the stacks were below the top of the inversion, their emissions might reach the ground because they might be trapped by a particularly strong inversion, or they might just reach the ground because of other atmospheric conditions. When the emissions are injected into the atmosphere high enough, they will be diluted enough to meet the letter of the law.
Rome happens to be in a nonattainment area for atmospheric particulate matter. That status somewhat limits the industrial development of this area. Our local newspaper, the Rome News-Tribune, does not like that. They have published editorials mocking the nonattainment status (like saying that our air seems clear enough to them).
To cast doubt on the legitimacy of the measurements that caused the nonattainment status, the editorial writer has pointed out that the air quality monitoring station is located near the base of the Plant Hammond stack. The implication seems to be that only an idiot would measure air quality that close to a pollution source, and thus the measurement must not be representative of Rome’s true air quality. I have written letters to them in the past pointing out that if you want to avoid measuring the emissions from a tall stack, the best place to put your instrumentation is at the very bottom of the stack. That is perhaps not intuitively obvious, but it is nevertheless true. However, the truth seems not to be a persuasive argument when it comes to commercial development and newspaper editorialists. (I might have mentioned this in an earlier post.)
You might be wondering why the top of the inversion is so much lower than the top of the mountain, where I have mentioned on several occasions that we are warmer than the surrounding lowlands because of a temperature inversion. The reason is that although the top of Lavender Mountain is above the actual inversion, the conditions that cause the inversion also work on the atmosphere up here. As the air on the mountaintop cools, it flows downhill into the lower areas, reinforcing the inversion down there. That air is replaced up here by the surrounding air, which is warmer than the air that flows down the mountain. I have mentioned before that we can be as much as 10 degrees F warmer than the air at the bottom of the mountain.
Oh, and Happy St. Patrick’s Day.
* The tropopause acts like the top of a temperature inversion. The air temperature gets lower as you go up in altitude until it reaches the tropopause. In the stratosphere, which is the layer above the troposphere, the air gets warmer as you go up. That’s why thunderstorms form anvil tops when they get to the tropopause. The clouds hit the warmer air and their buoyancy can’t get them any higher. At that point they tend to spread out sideways, forming the anvil top. Occasionally a very strong thunderstorm can push its clouds through the tropopause, but not very much higher.