An apparatus and method for measuring the liquid water content of a cloud or fog utilizes an environmentally controlled test chamber having a plurality of oppositely disposed windows selected to be optically transparent at a chosen wavelength of radiation. The windows of the chamber are positioned intermediate an infrared source and a wavelength scanning radiometer. An optical band pass filter is selected to pass radiation of a specific wavelength making the instrumentation substantially independent of cloud droplet size distribution enabling the measurement of only liquid water rather than water vapor.
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