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This work deals with the applicability of a star photometer to retrieve precipitable water vapor (W) at night-time by means of direct irradiance from stars using a 940 nm narrowband filter. Retrievals are assessed with simultaneous data from a Raman lidar system and linked with day-time values from a sun photometer using measurements taken from March to May 2007 at an urban site. Calibration of both the star and sun photometers was performed by the Modified Astronomical Langley Method. The retrieval of W from photometers is based on a look up table using a simplified expression for the water vapor transmittance and the relative optical water vapor air mass. This methodology presents a systematic uncertainty in W below 3% and 6% for sun and star photometry respectively. Retrievals of W from star photometry were 9% above those obtained from the Raman lidar technique, but with a great agreement in the temporal evolution from both instruments. The link of day- and night-time values of W using sun and star photometers for a more extended database showed a smooth continuity between consecutive periods. Read More
