You can calculate it as **Ri = Gr/Re^2**

where

Gr is the Grashof number

Re is the Reynolds number

or as Ri = [beta*g*(T_s - T_ref)*L]/V^2

where

g is the gravitational acceleration

beta is the thermal expansion coefficient

T_s is the (hot) surface temperature

T_ref is the reference temperature

L is the characteristic length

V is the velocity