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:

• Formula for humid air is taken from paper by P.T. Tsilingiris: 'Thermophysical and transport properties of humid air at temperature range between 0 and 100°C', 2007
• Formula for air is taken from paper by A. Dumas and M. Trancossi: 'Design of Exchangers Based on Heat Pipes for Hot Exhaust Thermal Flux, with the Capability of Thermal Shocks Absorption and Low Level Energy Recovery', 2009. They are calculated from polynomial curve fits to a data set for 100 K to 1600 K in the SFPE Handbook of Fire Protection Engineering, 2nd Edition Table B-2. You may find a free air property calculator from Pierre Bouteloup

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]
$\theta_{\mathrm{air}}$