2.4.3. Fanning Plume Shape

The third condition is called Fanning and now you see here that the adiabatic lapse rate and environmental lapse rate intersect but they intersect right below the source just below the source. These are all different scenarios and you can create any number of scenarios depending on where the source is and what is the lapse rate.

The Fanning plume shape occurs when the adiabatic lapse rate (the rate at which air cools with height under adiabatic conditions) and the environmental lapse rate (the actual temperature decrease in the atmosphere with height) intersect just below the pollutant source. This relationship is crucial because it affects how pollutants disperse in the atmosphere, and it signifies that thermal dynamics are at play in determining the plume’s behavior.

Imagine throwing a ball vertically. If you throw it upwards on a calm day, it rises and then falls back due to gravity. However, if you're on a windy day, the ball might get pushed side to side by the wind, creating an unpredictable path. Similarly, in the Fanning plume scenario, the temperature gradients influence whether the gases will rise or fall based on where they are relative to the mixing height.

So you can see that this is the mixing height. So the pollutant source is right above this mixing height. If a pollutant is released, it is following this line. And you see that in this particular instance, the temperature of the plume is less than that of the environment. So it tends to sink back to its source point, or it sinks back to the mixing height.

In the Fanning plume shape, once pollutants are released from their source, they generally follow a specific path. The temperature of the pollutants is lower than that of the surrounding air, which results in the plume sinking back towards the source or remaining close to the mixing height where it becomes stable. This interaction is key in understanding how pollutants behave in varying thermal conditions.

Think of a hot air balloon. If the air inside the balloon is warmer than the surrounding air, the balloon rises. But if it cools down and becomes less buoyant than the surrounding air, it stops rising and may even drift lower. Similarly, in the Fanning scenario, if the pollutant’s temperature cools too much relative to its surroundings, it won’t rise but instead descend back towards the source.

The reason it is called fanning is the z dispersion is now suppressed because the thermal adiabatic lapse rate and environmental lapse rate such that it does not allow the parcel to go up, but it can still disperse in the y direction.

In the Fanning condition, the vertical (z-axis) dispersion of pollutants is limited due to the balance between the adiabatic and environmental lapse rates. While the particles are constrained in vertical movement, they can still spread sideways (y direction), which describes the 'fanning' behavior. This limited vertical spread means that the concentration of pollutants can become significantly high near the source.

Visualize a hand fan: when you open a fan, the air flows outwards in a wide arc but doesn't shoot straight up. This spreading of air resembles how pollutants behave in the Fanning plume shape—they fan outwards horizontally but don’t rise much vertically.

Sometimes you can see this very clearly on certain days. In this case, you can see if you look at chimneys sometime, you will see a very straight plume like this. It is very rare to see it but sometimes you can see it if you are going in factories in certain seasons, probably.

The characteristics of the Fanning plume can be observed during specific weather conditions, particularly during temperature inversions. In such cases, you might notice that emissions from factory chimneys often appear as straight lines, extending horizontally without much vertical height. This is indicative of the limited vertical dispersion of pollutants under these conditions.

Picture a calm winter morning when cold air settles near the ground. If you observe cars or chimneys emitting smoke on such a day, the smoke may rise only slightly before leveling off, creating a 'flat-top' appearance. Just like how a pancake flattens in a pan, the pollutants flatten out instead of rising high.

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