A technique that utilizes radiative power from airborne night -thermal infrared imagery is described for monitoring Norris Geyser Basin in Yellowstone National Park. This technique does not define the total corrected radiative power emitted from hydrothermal areas. Instead, the uncorrected geothermal power for geohydrologic areas is compared to quantitatively document temporal changes in specific hydrothermal areas. Power -temperature histograms for geohydrologic areas illustrate hydrothermal characteristics of geohydrologic areas and document hydrothermal changes.