The embers in the forests are hard to be extinguished because of their smoldering combustion, such as concealing, long burning, invisible inspection and easy resurgence. To find the smoldering combustions timely and efficiently and to explore the characteristics and rules for the resurgences, a series of experiments have been presented in this study. With the help of a drone equipped with thermal infrared imaging system and meteorological data collection system, some experiments were conducted in the poplars in Nanjing Forest Police College, where ignitions were set, lighted, put out and resurged by human intervention. The embers were observed by the Infrared Radiometer on the drone in the daytime and at night. The results showed that the dispersion was bigger when the temperature of smoldering combustion was between 500～600 ℃, and the easier resurgence in the daytime than at night suggested that the higher external temperature the ember was, the easier resurgent it was. It was pointed that the temperature in forests was relatively stable in different plots and in different time, with a standard deviation of 1 and 9. The standard deviation of temperature data on the infrared images of embers varied from 30 to 85 and it revealed that the values between 80 and 85 would be suspected. In this method, the surroundings and parameter threshold of temperature in the smoldering combustions could be quantified to determine their eigen values of embers in the forests. It is reasonable for the researchers at this point to promote the development of the technology in the forest-fire prevention and the decision-makers to ask which results could provide some important methods and data for safe fire-fighting.