cross the Mississippi River from Principia College lies the Ameren Sioux Power Plant, home to a magnificent smokestack.

To some, this smokestack is considered an eyesore to an otherwise beautiful landscape. To others, it offers the ability to note how temperature and air stability affect the shape of smoke plumes.

Chemistry professor Simon Hanson says that smokestacks were created in order to prevent pollutants from creating ground level fog, which would reduce visibility and make it difficult to breathe. These tall structures shoot pollutants in the sky, above and away from the immediate, surrounding community.

When there is a smokestack with billowing clouds of smoke channeling out, the common reaction is disgust at the thought of how much its contributing to the pollution in the atmosphere. However, there are air filtration systems called scrubbers within the smokestack that remove more than 90 percent of a plant’s sulfur dioxide emissions. The majority of the gas that leaves the smokestack is just water vapor.

Depending on the time of day, the wind direction, the temperature, and the stability of the air, the clean exhaust reacts to the atmospheric elements which leads to the different smoke plumes. These smoke plumes can resemble fans or cones, and they can loop, loft, or fumigate the surrounding area.

“Fanning” refers to when the smoke spreads out horizontally like a fan, but does not expand vertically, so it appears to be a thin trail of smoke when seen head on from far away.

According to C. Donald Ahrens – author of “Meteorology Today: An Introduction to Weather, Climate, and the Environment” – fanning usually occurs in the early morning, when winds are light and a radiation inversion extends from the earth’s surface to well above the smokestack.

A radiation inversion is a stable environment where, if one had the ability to see air particles, they would expand due to heat as they rise in elevation. During the day, the earth absorbs infrared rays from the sun, warming up the planet. Once the sun goes down, the earth absorbs less and less and radiates ultraviolet rays back into the atmosphere, cooling the planet back down.

Fanning usually happens in the early morning because the ground is cold after the earth has radiated off heat throughout the night, so the air particles are at their coolest near the surface.

Fumigation is the next form a smoke plume might take as a result of later morning conditions. Fumigation occurs when an increased amount of volume in the air mixes with the smoke downward near the surface, but this occurs more commonly with shorter smokestacks because the layer of air near the surface becomes unstable as the ground warms. In an unstable atmosphere, as air particles rise from the surface, they begin to cool rather than warm.

The radiation inversion layer explained during fanning remains in place higher up above the unstable layer of air. If the top of the smokestack lies within the layer of unstable air and under the layer of stable air, the stable air prevents the smoke from traveling vertically and traps it, resulting in fumigation towards the surface. Fortunately, the Sioux Plant smokestack across the river is tall and fumigation is a rarity.

As the day goes on and if daytime heating continues, instability in the atmosphere increases. Air turbulence from strong winds can push the rising smoke up and down forming a wavy pattern known as looping.

Another form the smoke plume can take is that of a cone. This form is slightly complex and takes into consideration the adiabatic lapse rate of the gas within the smokestack, and the temperature of the surrounding air. The adiabatic lapse rate refers to the rate at which the temperature of a particle of air decreases with the increase in altitude.

When the adiabatic lapse rate of the gas coming out of the smokestack is equal to the temperature change of the air particles of the surrounding air, it is believed to be a neutral atmosphere and a smoke plume takes the shape of a horizontal cone as it exits the smokestack.

The final plume formation occurs after sunset. When the earth begins to radiate off more heat than it absorbs, the ground cools rapidly, bringing back into play the radiation inversion mentioned earlier. Unlike fumigation, the layer of stable air is now below the top of the smokestack; it extends from the earth’s surface to right below the top of the smokestack. Having the top of this layer of stable air below the top of the smokestack prevents the smoke released from mixing downward and causes it to float upward within the neutral air mass that was present during coning. This formation is called lofting.

As the sun begins to rise, radiation inversion expands above the top of the smokestack leading to the atmospheric conditions to support fanning, and the plume pattern begins again. Thus, smoke plumes can aid in measuring atmospheric stability.

Image courtesy of Julia Suber