Visual inspection of crude oil on water can determine the depth of thin layers of oil. However, catastrophic spills with millimeter (mm) thick oil will just be black, with no visual variation as the oil gets thicker. A day/night heat transfer model was developed to determine crude oil slick thickness. The model uses LWIR thermographic imagery, weather station outputs of air and water temperature, relative humidity, solar radiation, wind speed, other weather input such as cloud cover percent and altitudes, and measured thermal conductivity of Alaskan North Slope (ANS) crude oil. Outdoor field-testing was performed with fresh, weathered, and emulsified ANS crude oils that were placed on water at depths of 2-10 mm. A FLIR T640SC camera viewed the scene from a three-story roof-top to simulate small unmanned aerial vehicle (sUAV) altitudes. A low-cost, portable weather station was set up next to the pool and temperature calibrated LWIR imagery was collected every 15-minutes for 24-hours. The average oil surface temperature was measured for each target. The day/night model predicts oil slick thickness within one or two standard deviations. The fidelity of the thickness measurements is dependent on the accurate measurement of the atmospheric and weather parameters, sea state, heat transfer constants, and calibration and stability of the thermal camera.