瑞利数（Rayleigh number），是流体力学中的无量纲数，指自然对流和扩散热量、质量传递之比，计算式为：

其中：

In fluid mechanics, the Rayleigh number (Ra) for a fluid is a dimensionless number associated with buoyancy-driven flow, also known as free convection or natural convection. When the Rayleigh number is below a critical value for that fluid, heat transfer is primarily in the form of conduction; when it exceeds the critical value, heat transfer is primarily in the form of convection.