SAChE Atmospheric Dispersion (ELA967) Module 2 Practice Test

Increasing atmospheric instability means more turbulence and mixing in the air, which broadens a released plume. In Gaussian dispersion terms, sigma_y measures how spread out the plume becomes crosswind (horizontal), and sigma_z measures how tall the plume spreads (vertical). When instability rises, turbulent eddies are stronger and more efficient at mixing, so the plume spreads more in both directions. That’s why both sigma_y and sigma_z increase with instability. The other options imply less spreading, no change, or spreading only in one direction, which doesn’t fit the effect of enhanced turbulence on dispersion in both horizontal and vertical dimensions.