The Prandtl number is a dimensionless number, defined as the ratio of kinematic viscosity (also called momentum diffusivity) to thermal diffusivity.

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

Symbol
$\mathrm{Pr}_{\mathrm{gas}}$
Unit
-
Formulae
$4.969218948 \cdot 10^{-9} \theta_{\mathrm{air}}^{4} - 4.162785412 \cdot 10^{-7} \theta_{\mathrm{air}}^{3} + 2.240599044 \cdot 10^{-5} \theta_{\mathrm{air}}^{2} - 0.0003703124976 \theta_{\mathrm{air}} + 0.7215798365$humid air at 1 atm (Tsilingiris2007)
$\frac{c_{\mathrm{p_{\mathrm{gas}}}} \eta_{\mathrm{gas}}}{k_{\mathrm{gas}}}$general formula for gases
Related
$c_{\mathrm{p_{\mathrm{gas}}}}$
$\eta_{\mathrm{gas}}$
$k_{\mathrm{gas}}$
$\theta_{\mathrm{air}}$
Used in
$G_{\mathrm{encl}}$
$\mathrm{Nu}_{\mathrm{c}}$
$\mathrm{Ra}_{\mathrm{c}}$
$\mathrm{Ra}_{\mathrm{encl}}$
$\mathrm{Ra}_{\mathrm{L}}$