Rayleigh number

The Rayleigh number is a dimensionless number defined as the product of Grashof and Prandtl numbers. For free convection near a vertical wall, this number is

Rax,c=Grx,cPr =  g β (Ts-T∞)x3
                ν α

where

Ra   Rayleigh number
Gr   Grashof number
Pr   Prandtl number
g    gravity
Ts   Temperature of surface
T∞     Quiescent temperature
ν    kinematic viscosity
α    thermal diffusivity

The Rayleigh number is named after Lord Rayleigh.