Derating at altitude refers to the reduction in a heat sink’s cooling capability as elevation increases. This occurs because air density decreases with altitude. Convective heat transfer depends on the mass of air moving past the fins. Thinner air carries less heat away. For natural convection, performance can degrade significantly above 2000 meters. For forced convection, while the mass flow rate drops, the fan’s constant volumetric flow (CFM) can partially compensate, but performance still decreases. Manufacturers often provide derating curves. A rule of thumb is to select a heat sink with 10-20% lower Rθ (better performance) than calculated at sea level if the device will operate at high altitudes.