In heat transfer analysis, thermal diffusivity is the ratio of thermal conductivity to heat capacity.

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

where k is thermal conductivity and ρc_p is heat capacity. Units are m²/s.

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

See also thermodynamics.