Temperature is probably the parameter that most people associate with when they are introducted to the term heat flow. More importantly then temperature itself though is the change in temperature over a given distance. If we consider two points, \(T_1\) and \(T_2\), over a given distance, \(\Delta z\), the thermal gradient is defined as \(\frac{\delta T}{\delta z} = (T_2 - T_1) / \Delta z\). In other words, for geothermal purposes, the change in temperature with depth. Although thermal gradient exists as a vector comoponent in three dimensional space, if we assume the surface of the Earth to be a relatively flat, insulating layer (a reasonable assumption), then the solution to determining thermal gradient within the Earth can be reduced in most cases to only the vertical direction. SI units for thermal gradient are \(^\circ K/m\), however, the official stance of the International Heat Flow Commission designates that thermal gradient be reported in \(^\circ C/km\). While the unit change from \(^\circ K\) to \(^\circ C\) obvisouly has no effect on the resulting value, the change from \(m\) to \(km\) scales output to more intuitive values given the relatively low thermal gradients found across the globe.