We describe an experimental method to measure the water density of a plume created from a shallow underwater explosion. Our approach is based on the attenuation characteristics of a microwave signal propagated through the plume. A unique correlation exists between the amount of attenuation and macroscopic quantities including the volume fraction of water in air. In this article, both the theory required for data analysis and the experimental arrangement used for measuring the highly attenuated microwave signal are discussed. ©1997 American Institute of Physics.