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

<math>\alpha = {k \over {\rho c_p}}</math>

where:

• <math>k\,</math> : thermal conductivity (SI units : W/(m K) )
• <math>\rho c_p\,</math> : volumetric heat capacity (SI units : J/(m³K) )
• <math>\rho\,</math> : density (SI units : kg/(m³) )
• <math>c_p\,</math> : specific heat capacity (SI units : J/(kg K) )

The SI units for thermal diffusivity is m²/s.

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, <math>\alpha\,</math> ~ 10^-6 m²/s.

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

• Thermodiffusion
• Heat capacity
• Specific heat capacity
• Heat equation
• Thermal conductivity
• Thermal effusivity
• Thermal time constant

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