The Rayleigh number above is for convection in a bulk fluid such as air or water, but convection can also occur when the fluid is inside and fills a porous medium, such as porous rock saturated with water. Then the Rayleigh number, sometimes called the Rayleigh-Darcy number, is different. In a bulk fluid, i.e., not in a porous medium, from the Stokes equation, the falling speed of a domain of size  of liquid . In porous medium, this expression is replaced by that from Darcy's law , with  the permeability of the porous medium. The Rayleigh or Rayleigh-Darcy number is then
This also applies to A-segregates, in the mushy zone of a solidifying alloy.