# Rayleigh number

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## Revision as of 03:39, 21 September 2005

The Rayleigh number is the ratio of the buoyancy force to the viscous force in a medium.

$Ra = \frac{g \alpha \Gamma d^4}{\nu \kappa}$

where $g =$ acceleration due to gravity $\alpha =$ coefficient of thermal expansion $\Gamma =$ background temperature gradient $d =$ length scale of flow $\nu =$ kinematic viscosity $\kappa =$ thermal diffusivity