This paper addresses practical issues related to the processing of 1-km National Oceanic and Atmospheric Administration (NOAA) advanced very high resolution radiometer (AVHRR) data for producing a consistent, long-term time series of active fire locations over the Continental United States and Canada. The effects of the interannual changes in measured background temperatures, caused by the orbital drift of the afternoon NOAA satellites and by environmental factors, are investigated. Background temperature changes are analyzed using a time series of monthly mean clear-sky brightness temperatures from the NOAA National Environmental Satellite, Data, and Information Service (NESDIS) Pathfinder Atmosphere (PATMOS) data set at a 1° x 1° resolution. Examples of target areas over four predominant land cover types, as defined in the International Geosphere-Biosphere Programme (IGBP) global 1 km data set, are presented. The results indicate that over forests (defined as >60% tree canopy cover) the contrast between nonburning background and fire pixels is nearly always sufficient for successful fire detection. Over nonforested areas, however, the low saturation temperature of the mid-IR channel on the NOAA 7 to NOAA 14 satellites often sets a physical limit to the separation of valid fire pixels and false ones. Moreover, the severity of this effect changes over the years with the changing background temperatures. The results suggest that because of the potential spurious trends in the number of fires, nonforested areas be excluded from the multiyear analysis. However, a detailed assessment of the emissions from nonforest fires is needed to quantify the effect of this on continental-scale emission estimates.