A heat sink usually consists of a base with one or more flat surfaces and an array of comb or fin-like protrusions to increase the heat sink's surface area contacting the air, and thus increasing the heat dissipation rate. While a heat sink is a static object, a fan often aids a heat sink by providing increased airflow over the heat sink—thus maintaining a larger temperature gradient by replacing the warmed air more quickly than passive convection achieves alone—this is known as a forced-air system.