The formula is taken from IEC 60287-2-3

Symbol
$\nu_{\mathrm{air}}$
Unit
m²/s
Formulae
$9.5 \cdot 10^{-8} \theta_{\mathrm{at}} + 1.32 \cdot 10^{-5}$
Related
$\nu_{\mathrm{gas}}$
$\theta_{\mathrm{at}}$
Used in
$C_{\mathrm{vair}}$
$\mathrm{Gr}_{\mathrm{prot}}$
$\mathrm{Re}_{\mathrm{air}}$