There is a non-linear, diminishing returns relationship between airflow (in Cubic Feet per Minute, CFM or m³/h) and heat sink thermal resistance (Rθ_sa). Increasing airflow initially yields a large improvement in cooling as it disrupts the boundary layer of hot air on the fins. However, as airflow becomes very high, the rate of improvement slows; the thermal resistance curve flattens. Doubling the airflow does not halve the thermal resistance. The relationship is plotted on performance graphs. When selecting a fan, it’s important to match its Pressure-Flow (P-Q) curve to the heat sink’s flow impedance. A high-CFM fan with low static pressure may stall on a dense heat sink.