1. (Or geothermal gradient.) According to Smithsonian Physical Tables, the rate of variation of temperature in soil and rock from the surface of the earth down to depths of the order of kilometers. It varies greatly from place to place, depending on the geological history of the region, the radioactivity of the underlying rocks, and the conductivity of the upper rocks. An average is about +10°C per km. 2. Same as temperature gradient. See also lapse rate.