In addition to heat conduction in materials through collisions of internal particles, heat can also be transferred through the direct exchange of matter.
This process is called convection.
A familiar example can be observed when sunlight enters through a window above a heated radiator.
The air warmed by the radiator rises due to its lower density, while cooler air flows in from below.
The resulting density fluctuations change the refractive index, which becomes visible as a shimmering effect in the air.
In heat sinks, which are usually made of thermally conductive metals, this effect is used to cool electronic components.
Heat is transferred to the fins of the heat sink, which then warm the surrounding air.
The thermal conductivity of air alone would be far too low to transport away the heat.
However, by rising warm air and the inflow of cooler surrounding air, heat dissipation is significantly increased.
By adding fans, convection can be further enhanced, allowing in practice even several hundred watts of waste heat to be cooled relatively easily.