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.