In heat transfer analysis, thermal diffusivity (but note that the symbols,, and are all commonly used) is the thermal conductivity divided by the volumetric heat capacity. It has the SI unit of m²/s. where: : thermal conductivity : density (kg/m³) : specific heat capacity (J/) The denominator of the thermal diffusivity expression above,, can be identified as the volumetric heat capacity with the SI unit of J/(m³·K). Substances with high thermal diffusivity rapidly adjust their temperature to that of their surroundings because they conduct heat quickly in comparison to their volumetric heat capacity or 'thermal bulk' and they generally do not require much energy from their surroundings to reach thermal equilibrium.