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

The formula is taken from IEC 60287-2-3

Symbol
$\mathrm{Pr}_{\mathrm{air}}$
Unit
-
Formulae
$- 0.00025 \theta_{\mathrm{at}} + 0.715$
Related
$\mathrm{Pr}_{\mathrm{gas}}$
$\theta_{\mathrm{at}}$
Used in
$C_{\mathrm{vair}}$
$\mathrm{Nu}_{\mathrm{prot}}$
$\mathrm{Ra}_{\mathrm{prot}}$