Rayleigh number

The Rayleigh number is a dimensionless quantity used in fluid mechanics to predict the onset of convection in a fluid. It is a combination of parameters including density, gravity, thermal expansion, depth, temperature difference, viscosity, and thermal diffusivity.1

Specifically, when a fluid is heated from below and cooled from above, the Rayleigh number helps determine if convection will occur. If the Rayleigh number is below a critical value of 1,708, the fluid remains stable. However, if it exceeds this threshold, convective rolls, known as Bénard cells, will form. This phenomenon is relevant to understanding atmospheric and oceanic circulation. The Richardson number, a related parameter, is used to predict fluid turbulence and the destruction of density currents.12