Expresses the ratio of the destabilising effects of buoyancy to the stabilising effects of diffusion of heat and momentum. Explicitly, Ra=geta Delta T H^3/( ukappa) in which g is the acceleration due to gravity, eta is the thermal expansion coefficient, Delta T is the temperature difference, h is the vertical length scale, u is the kinematic viscosity, and kappa is the thermal diffusivity. The Rayleigh number is usually used to classify the stability of a fluid column heated from below, convection resulting when some relatively small value is exceeded.