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# Thermal diffusivity

In heat transfer analysis, thermal diffusivity (symbol: $\kappa\,$, but note that the symbols α, D, and k are all commonly used) is the ratio of thermal conductivity to volumetric heat capacity.

$\kappa = {k \over {\rho c_p}}$

where: $k\,$ is thermal conductivity (SI units: watts per metre-kelvin, W∙m-1∙K-1) $\rho c_p\,$ is the volumetric heat capacity (density kg∙m-3 times specific heat capacity J∙kg-1K-1; SI units: joules per cubic metre-kelvin, J∙m-3∙K-1)

The SI units for thermal diffusivity are square metres per second or m2∙s-1.

Substances with high thermal diffusivity rapidly adjust their temperature to that of their surroundings, because they conduct heat quickly in comparison to their thermal 'bulk'.

For common rock material, $\kappa\,$ ~ 10-6 m2∙s-1.

Thermal diffusivity of air at 300 K is 0.000024 m2/s.

For a plot of thermal diffusivity of air as a function of absolute temperature see James Ierardi's Fire Protection Engineering Site