A Simple Instrument and Technique for Measuring Columnar Water Vapor via Near-IR Differential Solar Transmission Measurements

This paper describes a simple two-channel solar radiometer and data retrieval technique for sensing the columnar content of atmospheric water vapor via differential solar transmission measurements in and adjacent to the 940-nm water vapor absorption band. The instrument features two parallel channels for simultaneous measurements in and out of the absorption band to eliminate temporal variability effects in the differential comparison of the data from the two channels. The water vapor transmittance is determined by a modified Langley plot analysis of the ratio of the two channel signals. A statistical band model which closely follows the square-root law is then used to extract the columnar water vapor amount from the water vapor band transmittance. Error analyses and experimental results indicate that the instrument/technique can be reasonably employed to retrieve water vapor amounts with an error of 10% or less.