Prandtl number is a dimensionless parameter that reperesents the ratio of the momentum and thermal diffusivities.
Where
Prandtl number is 0.7 for air.
A Prandtl Number less than 1 generally means that the thermal boundary layer has a higher thickness than the velocity boundary layer. If the number is greater than 1 then the opposite is true.
If you have less than 0.7 pringles number you die