How do dispersion coefficients σ_y and σ_z depend on stability and distance?

How do dispersion coefficients σ_y and σ_z depend on stability and distance?

• A. They depend only on wind speed.
• B. They decrease with distance and are larger for stable conditions.
• C. They increase with downwind distance and are larger for unstable conditions.
• D. They are constant with distance.

Answer: (C)

Dispersion coefficients describe how a plume spreads crosswind and vertically as it travels downwind. They grow with downwind distance because turbulence has more time to mix the plume, so the spreading increases the longer the plume travels. The level of turbulence is set by atmospheric stability: unstable conditions (buoyant, convective) create stronger mixing and larger σ_y and σ_z, while stable conditions suppress turbulence and lead to smaller dispersions. Wind speed matters for transport, but the primary factors governing the growth of σ_y and σ_z are distance and stability, not a sole dependence on wind speed. So these coefficients increase with distance and are larger under unstable conditions, which matches the statement.

Dispersion coefficients describe how a plume spreads crosswind and vertically as it travels downwind. They grow with downwind distance because turbulence has more time to mix the plume, so the spreading increases the longer the plume travels. The level of turbulence is set by atmospheric stability: unstable conditions (buoyant, convective) create stronger mixing and larger σ_y and σ_z, while stable conditions suppress turbulence and lead to smaller dispersions. Wind speed matters for transport, but the primary factors governing the growth of σ_y and σ_z are distance and stability, not a sole dependence on wind speed. So these coefficients increase with distance and are larger under unstable conditions, which matches the statement.