The Grashof number is the ratio of the buoyancy forces to the viscous forces:

Gr = [g*beta*(T_s - T_infinity)*L^3]/nu^2

where

g is the gravitational acceleration

beta is the thermal expansion coefficient

T_s is the surface temperature

T_infinity is the free-stream temperature

L is the characteristic length

nu is the kinematic viscosity

and the Prandtl number is the ratio of the momentum diffusivity to the thermal diffusivity:

Pr = nu/ alpha

where

nu is the kinematic viscosity

alpha is the thermal diffusivity