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:

Symbol
$\alpha_{\mathrm{gas}}$
Unit
m2/s
Formulae
$- 6.369279936 \cdot 10^{-14} \theta_{\mathrm{air}}^{4} - 5.769352751 \cdot 10^{-12} \theta_{\mathrm{air}}^{3} + 2.373056947 \cdot 10^{-10} \theta_{\mathrm{air}}^{2} + 1.161914598 \cdot 10^{-7} \theta_{\mathrm{air}} + 1.847185729 \cdot 10^{-5}$humid air at 1 atm (Tsilingiris2007)
$9.1018 \cdot 10^{-11} T_{air}^{2} + 8.8197 \cdot 10^{-8} T_{air} - 1.0654 \cdot 10^{-5}$air at 1 atm (Dumas&Trancossi2009)
$1.5556 \cdot 10^{-10} T_{air}^{2} + 4.119 \cdot 10^{-8} T_{air} - 4.3274 \cdot 10^{-6}$air at 1 atm (UW/MHTL 8406, 1984
$\frac{k_{\mathrm{gas}}}{c_{\mathrm{p_{\mathrm{gas}}}} \rho_{\mathrm{gas}}}$general formula for gases
Related
$c_{\mathrm{p_{\mathrm{gas}}}}$
$k_{\mathrm{gas}}$
$\rho_{\mathrm{gas}}$
Gas density [kg/m³]
$\theta_{\mathrm{air}}$