Precipitable Water Measurements with Sun Photometers

Abstract

This paper presents a brief history of spectroscopic and radiometric methods of remotely sensing precipitable water. We demonstrate a new method of calibrating sun photometers with microwave radiometer data to obtain precipitable water, and a new expression is derived for obtaining precipitable water with the 940 nm band that is valid for 1.1?3.5 cm of precipitable water in the path toward the sun. Also, we illustrate the dependence of water-vapor calculations on optical filter bandwidth and on the specific water-vapor band chosen. We present time histories of precipitable water to show the variabilities that can occur. Comparisons between sun photometer and microwave radiometer data for nearly identical atmospheric paths nearly always agree to within ±5%.