The formula is taken from IEC 60287-2-3

Symbol
$k_{\mathrm{air}}$
Unit
W/m.K
Formulae
$7.2 \cdot 10^{-5} \theta_{\mathrm{at}} + 0.0242$
Related
$k_{\mathrm{gas}}$
$\theta_{\mathrm{at}}$
Used in
$C_{\mathrm{vair}}$