In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy, and is approximately analogous to whether a material is "cold to the touch". It has the SI unit of m²/s. Thermal diffusivity is usually denoted α but a, κ, K, and D are also used. The formula is: