How does terrain (hills, valleys) affect atmospheric dispersion?

How does terrain (hills, valleys) affect atmospheric dispersion?

• A. Terrain has no effect.
• B. Terrain only affects deposition.
• C. Terrain alters wind flow, channeling, turbulence, and mixing heights.
• D. Terrain always reduces concentrations.

Answer: (C)

Terrain changes how air moves and mixes, which is what governs how pollutants disperse. Hills, valleys, and ridges reshape wind fields by channeling flow along slopes and through passes, creating wakes and altered directions that can carry a plume farther in certain directions or trap it in a basin. Rough terrain also generates mechanical turbulence, increasing mixing in some situations, while sheltered lee regions can produce low winds and stagnation that limit dilution. Importantly, terrain modifies the mixing height—the depth of the well-mixed layer—so vertical dilution can be enhanced on exposed slopes or reduced in valleys during stable conditions, leading to higher near-surface concentrations in those cases. Because of these multiple effects, terrain does not simply affect deposition, nor does it always reduce concentrations; it can either increase or decrease dispersion depending on wind, stability, and topography.

Terrain changes how air moves and mixes, which is what governs how pollutants disperse. Hills, valleys, and ridges reshape wind fields by channeling flow along slopes and through passes, creating wakes and altered directions that can carry a plume farther in certain directions or trap it in a basin. Rough terrain also generates mechanical turbulence, increasing mixing in some situations, while sheltered lee regions can produce low winds and stagnation that limit dilution. Importantly, terrain modifies the mixing height—the depth of the well-mixed layer—so vertical dilution can be enhanced on exposed slopes or reduced in valleys during stable conditions, leading to higher near-surface concentrations in those cases. Because of these multiple effects, terrain does not simply affect deposition, nor does it always reduce concentrations; it can either increase or decrease dispersion depending on wind, stability, and topography.