The thermal diffusivity is the thermal conductivity divided by density $\rho$ and specific heat capacity at constant pressure $c_p$. 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".
The sources are:
| $1.847185729{\cdot}{10}^{-5}+1.161914598{\cdot}{10}^{-7} \theta_{gas}+2.373056947{\cdot}{10}^{-10} {\theta_{gas}}^2-5.769352751{\cdot}{10}^{-12} {\theta_{gas}}^3-6.369279936{\cdot}{10}^{-14} {\theta_{gas}}^4$ | humid air @ 1 atm (Tsilingiris2007) |
| $9.1018{\cdot}{10}^{-11} {T_{gas}}^2+8.8197{\cdot}{10}^{-8} T_{gas}-1.0654{\cdot}{10}^{-5}$ | air @ 1 atm (Dumas&Trancossi2009) |
| ${10}^{-6}\left(-4.3274+4.119{\cdot}{10}^{-2} T_{gas}+1.5556{\cdot}{10}^{-4} {T_{gas}}^2\right)$ | dry air @ 1 atm (UW/MHTL 8406, 1984) |
| $\frac{k_{gas}}{\rho_{gas} c_{p,gas}}$ | general formula for gases |
