## 4 Results
### Temperature Trends  
Figure 3a shows the differences between the mean air temperature measured at the landfill and the mean LST over the entire landfill recorded by satellites within the same hour as the air temperature. The differences show that the LST is consistently higher than the air temperature. Although the LST is expected to exceed air temperature somewhat as a normal result of subsurface processes in the landfill, differences that systematically increase over time indicate changes in the subsurface activities such as a smoldering or fire event. The temperature variation shown in Figure 3a exhibits a periodic cycle due to seasonal changes, whereby the LST is higher during the summer or late spring and lower during the winter or late fall. The landfill maintains an average mean difference of approximately 6 \({}^{\circ}\)C (black straight line labeled \"Mean 2000-2016\") between 2000 and 2016. From 2000 to 2011, the mean difference varies about 6 \({}^{\circ}\)C. After 2013, the mean difference appears to increase. Figure 3b shows the difference between the highest and lowest LSTs at the landfill (LST_max, LST_min). This difference shows an increasing trend similar to that of the mean difference, with relatively consistent values from 2000 to 2011, and higher values afterward. The higher temperature differences correspond to observed subsurface thermal events at the landfill. Figure 3b can shed light on some of the anomalies in the temperature change in the landfill as the difference between the lowest and the highest LSTs measured by remote sensing on the same day is increasing. However, this difference cannot tell us where the change is happening. With the extrapolation of LST data from the satellite images, it is possible to use the temperature trends to independently identify a recent SSE [21]. To determine the locations of hotspots, three areas were selected for closer study (with a focus on the spatial temperature distribution): the South Quarry (red) in the south area of the landfill, where most fire incidents have occurred and where the maximum temperature was recorded (both satellite and ground measurement); the Neck area (blue) to track any hotspots approaching the neck; and the North Quarry (green) adjacent to the radioactive waste area in the north part of the landfill. In Figure 4a, the landfill is divided into regions, using the mean LST as a baseline, to enable comparison of the deviations of these three different regions from the mean landfill temperature over 16 years. Figure 4 also depicts the history of heat elevation of the landfill during the study period. It reveals that during the 2000-2011 period, the all-regions temperature is clustered around the mean landfill temperature except for that of the upper region, which experienced elevated temperatures during the years 2004-2006 (Figure 4b in the lower middle part of Figure 4 shows the moving average). Figure 4: Relative temperature differences in the 17-year span for Bridgeton Sanitary Landfill, Missouri. (**a**) Mean LST of different regions within the landfill; (**b**) LST generated from moving average approach. Figure 3: (**a**) Comparison of mean LSTs and mean air temperatures and (**b**) a comparison of the maximum and minimum LSTs for given image dates for Bridgeton landfill. Starting in 2013, compared to previous years, the whole landfill exhibited elevated temperatures. The lower region (South Quarry) continued to have the highest temperatures from 2013 to 2015. However, the northern region, which had maintained consistently low temperatures, experienced an increase in temperature of approximately 7 \({}^{\circ}\)C between 2013 and 2016 compared to the southern region. This can be clearly observed in the spike of elevated temperature in mid-2014 when the temperature of the North Quarry (green line in Figure 4) rises above the mean temperature of the landfill. The subsurface temperature in the southern area of the landfill continued to increase after 2011.