The question asks to identify the atmospheric condition corresponding to a "fanning" type plume from a tall stack.

Fanning Plume Explained

A "fanning" plume is characterized by significant horizontal dispersion downwind but minimal vertical dispersion. This shape occurs when the plume spreads out like a fan, staying relatively contained vertically.

Atmospheric Stability and Plume Types

The shape of an air pollution plume is primarily determined by the atmospheric stability, which is related to the lapse rate (the rate at which temperature decreases with altitude).

• Unstable conditions (strong lapse rate): Lead to significant vertical mixing, resulting in "looping" or "coning" plumes.
• Stable conditions (weak lapse rate or inversion): Restrict vertical motion, leading to horizontal spreading.

Inversion Condition

A temperature inversion occurs when the normal lapse rate is inverted, meaning the temperature increases with altitude in a specific layer. This creates a very stable atmosphere.

• In such a stable environment, vertical movement of air is strongly suppressed.
• When a plume is released from a tall stack into or below an inversion layer, the stable air prevents it from rising or dispersing vertically.
• Consequently, the plume is forced to spread horizontally, creating the characteristic "fanning" shape.

Therefore, a fanning plume indicates that the atmosphere is under the condition of inversion, signifying high atmospheric stability.

Conclusion

The fanning plume type directly implies that the atmosphere is under an inversion condition, which causes significant atmospheric stability and restricts vertical pollutant dispersion.