Molecules in the atmosphere absorb differently in different spectral regions. In particular, atmospheric moisture affects transmission in certain regions. Results are presented of a study on the effects of atmospheric moisture on temperature discrimination by infrared detectors. Probability of accepting a low temperature object and of rejecting one 1000 K higher was calculated. Detectors were in two bands in the middle infrared (IR) region. Even for transmission distances as short as 100 feet, probabilities were strongly affected by varying temperature and relative humidity.