什么是 Heat flow？

The heat flow (or heat flux) at a place is the power that leaves the Earth’s interior through each square metre of its surface there. It is measured in milliwatts per square metre ($\mathrm{mW}/\mathrm{m}^{2}$).

The heat flow $q$ at a point of the Earth’s surface, or surface heat-flow density, is the power of internal origin that crosses the surface there, per unit area, counted positive outwards. It is measured in $\mathrm{W}\,\mathrm{m}^{-2}$, in practice in $\mathrm{mW}\,\mathrm{m}^{-2}$. Integrated over the whole surface, it gives the Earth’s heat loss $Q = \oint q\,\mathrm dS$.