Microwave Emissivity Measurements of Bubbles and Foam

Measurements were made in the laboratory of the emissivities of a variety of bubble and foam samples at a wavelength of 3.2 cm. The technique was an active one in which the absorptivities of the samples were determined. Both free-space and waveguide measurements were made, and emissivities were found to be near 0.6 for some single-layer bubble rafts and near 1.0 for mixed foams of 3- to 4-mm thickness. These measurements support the previously expressed belief that it should be possible to infer sea state and approximate wind speed from passive microwave measurements interpreted in percentage of foam coverage.