A Partial Aperture Calibration (PAC) scene will occur at the point in the orbit when the satellite has just come out of the Earth's shadow and into the sunlight but is still flying over "dark" Earth. This position causes a glint of sunlight to reflect off one of four quartz facets and into the aperture of the sensor. This glint will cause an elongate bright oval (candle flame) in an otherwise normal night scene.
The PAC glint appears only in the reflective bands. Because the thermal band is unaffected, night scenes with the PAC glint in them are still valid scenes for interpretation. Cloud cover will be listed as "-1" for PAC scenes (as with all night scenes), even though that may not be the case.