The heat transfer from the heat sink occurs by three mechanisms. First convection of the surrounding air, second, conduction through the air, and third the radiation. All of these depend on the geometry, material selection and boundary conditions. Heat transfer by radiation is a function of both the heat sink temperature, and the ambient temperature.
In a normal finned design, both conduction and convection are important. However in n situations where convection is low, such as a flat heat sinks (non-finned panel with low airflow), then radiative cooling can be the important factor as well. In any case, an important design factor is the surface properties. Black painted matte surfaces radiate with much higher efficiently than bare metals.