= Rayleigh number
{c}
{title2=$\operatorname{Ra}$}
{wiki}
The Rayleigh number compares buoyancy driving with viscous and thermal diffusion. For a layer of thickness $h$, temperature drop $\Delta T$, density $\rho$, thermal expansion coefficient $\alpha_T$, gravitational acceleration $g$, thermal diffusivity $\kappa$, and dynamic viscosity $\mu$,
$$
\operatorname{Ra}=\frac{\rho g\alpha_T\Delta T h^3}{\mu\kappa}.
$$
Thermal convection begins when this exceeds a geometry-dependent critical value.
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