<Li> ν is the kinematic viscosity, and </Li> <Li> L is characteristic length . </Li> <P> The Rayleigh number can be understood as the ratio between the rate of heat transfer by convection to the rate of heat transfer by conduction; or, equivalently, the ratio between the corresponding timescales (i.e. conduction timescale divided by convection timescale), up to a numerical factor . This can be seen as follows, where all calculations are up to numerical factors depending on the geometry of the system . </P> <P> The buoyancy force driving the convection is roughly g Δ ρ L 3 (\ displaystyle g \ Delta \ rho L ^ (3)), so the corresponding pressure is roughly g Δ ρ L (\ displaystyle g \ Delta \ rho L). In steady state, this is canceled by the shear stress due to viscosity, and therefore roughly equals μ V / L = μ / T c o n v (\ displaystyle \ mu V / L = \ mu / T_ (conv)), where V is the typical fluid velocity due to convection and T c o n v (\ displaystyle T_ (conv)) the order of its timescale . The conduction timescale, on the other hand, is of the order of T c o n d = L 2 / α (\ displaystyle T_ (cond) = L ^ (2) / \ alpha). </P>

Ratio of energy transferred by convection to that by conduction