Passive infrared remote detection of hazardous gases, vapors, and aerosols is based on the difference, Δ T, between the air temperature of the threat vapor cloud and the effective radiative temperature of the background. In this paper I address the problem of detection with a low-angle-sky background. I used Modtran to predict Δ T and atmospheric transmittance for standard atmospheric models. The detection limits, at 2-cm(-1) resolution, are discussed for sulfur hexafluoride, Sarin, trichloroethylene, methyl isocyanate, mustard gas, methyl chloride, and sulfur dioxide for selected cases with the U.S. Standard, the Subarctic Winter, and the Tropical models. I used a particularly interesting case of Sarin detection with the Subarctic Winter atmospheric model to illustrate the power of Modtran to predict subtle changes in Δ T with angle of elevation (AOE).