concept

Nusselt number

The ratio of the heat convection actually carries to the heat plain conduction would carry through the same layer. One means convection is adding nothing; thirty means the flow is moving heat thirty times faster. For a planetary mantle it scales roughly as the cube root of the Rayleigh number, and that makes a thermostat: a hotter mantle is less viscous, so it convects harder, so it cools faster — until it settles back down. A planet regulates its own rate of heat loss.