1990. Total column water vapor is measured at 940 nm and 830 nm (reference wavelength) using the same sun photometer first used on February 4, 1990. The ratio of these two wavelengths provides the total water vapor. The trend is slightly down (approx. -1.5 mm/decade). Calibration: NOAA GPS data from Galveston, TX, and Mauna Loa Observatory, HI. Measurements are made at or near solar noon when clouds are not before the sun. Water vapor is the key global warming gas. The 1997-98 peak in Mims's data occurred during a major El Nino. No such peak occurred during the 2015-16 El Nino. The general shape of the data resembles the global water vapor plot in NASA's ongoing NVAP study. In addition to Mims' measurements in his home state of Texas, he made atmospheric measurements in Brazil during two three-week campaigns for NASA’s Goddard Space Flight Center. In August 1995, Mims led a 2-man team to measure the ozone layer during the SCAR-B campaign at Cuiaba in central Brazil since the ozone instrument aboard the Nimbus-7 satellite had ceased working. In August 1997, Mims led a 2-man team that measured ozone layer, smoke optical depth, UV-B and water vapor near Alta Floresta in Amazonia. During the fall of 1996, NASA’s Goddard Space Flight Center assigned Mims to fly at a moment's notice to a series of 7 major forest fires in Utah, California, Wyoming and Montana. A GSFC scientist had discovered that a new satellite ozone instrument could also detect smoke, and Mims was assigned to measure total ozone and the optical depth of smoke during satellite overpasses. Twilight Science Mims’ latest research involves the measurement of the altitude of aerosol layers in the atmosphere using a new kind of twilight photometer. A twilight photometer is a highly sensitive light meter that is pointed at the zenith sky for up to an hour after sunset or before sunrise. As the sun sets in the evening, the edge of Earth’s shadow over a fixed point rises overhead. (The opposite occurs before sunrise.) Particles in the sunlit atmosphere just above Earth’s shadow scatter sunlight toward the surface, where it can be detected by a twilight photometer. The elevation of these particles can be calculated. Twilight photometry traditionally uses a telescope pointed toward the zenith that focuses very dim twilight onto a photodiode or photomultiplier tube connected to multi-stage amplifier. Mims’ design is unique in