The traditional (or heritage) method of detecting fog/low clouds is a brightness temperature difference (BTD) product. The difference between brightness temperatures at 10.7 µm and 3.9 µm highlights water-based clouds because those clouds do not emit as a blackbody at 3.9 µm, so the inferred temperature (computed assuming a blackbody emission) is colder than that temperature computed using 10.7 µm radiation, because clouds emit radiation at 10.7 µm more like a blackbody.
At night, the GOES-R IFR Probabilities use a pseudo-emissivity at 3.9 µm in lieu of the 3.9-11µm BTD to highlight regions of water-based clouds at low levels. The 3.9 µm Pseudo-emissivity is the ratio of the observed radiance at 3.9 µm to a computed 3.9 µm blackbody radiance that is based on the observed 10.7 µm brightness temperature. In other words, the observed 10.7 µm brightness temperature is computed from the 10.7 µm radiance. That computed brightness temperature is then used to compute a 3.9 µm radiance that would be detected if the emitter was a blackbody.
The 3.9 µm pseudo-emissivity produces a satellite signature for low water clouds similar to the brightness temperature difference but is used instead because it is less sensitive to scene temperature. Skill scores for fog/low stratus detection are higher when the pseudo-emissivity is used in the algorithm to find regions of low clouds/fog than when the brightness temperature difference field is used.