What factor is critical in determining the effective height of a stack emission?

The effective height of a stack emission is significantly influenced by the temperature difference between the gases being emitted from the stack and the surrounding air. When gases are emitted at a higher temperature, they tend to rise more than cooler gases due to their reduced density. This buoyancy effect allows the emissions to ascend further into the atmosphere, dispersing pollutants over a larger area and reducing ground-level concentrations. The higher the temperature difference, the greater the rise, which contributes to better dispersion of pollutants and thus is critical for determining effective stack height.

In contrast, while the diameter of the stack can influence the flow rate and distribution of the emissions, it does not directly affect the height the emissions reach. The amount of pollutants emitted is also relevant but serves more as a measure of potential impact rather than determining how high the emissions can rise. Local topography can influence airflow patterns and dispersion but does not directly control the effective height of emissions from the stack itself. Thus, the temperature difference is the key factor that directly affects how high the emissions travel into the atmosphere.