Aircraft observations of the microwave emission from the Bering Sea show that the spectral character of brightness temperature variation with wind speed in a fully developed sea is due to a combination of surface roughness and a surface layer with varying dielectric constant over a physical depth of a few millimeters. The varying dielectric layer is identified with cusps formed by the bubbles associated with thin foam streaks. Computations of the radiative transfer through such a layer show that the change in microwave emissivity due to the presence of that layer decreases as microwave wavelength increases. Since foam streak coverage on the ocean surface increases with surface wind speed, the increase in microwave brightness temperature with increasing wind speed will be less at longer wavelengths, a characteristic that is observed in the experimental results presented. In addition, it has been observed that in fetch-limited situations the microwave brightness temperature depends in the same way on surface wind speed but is essentially independent of varying foam coverage, the implication being that the surface wave structure becomes more important in determining the microwave brightness temperatures as fetch approaches zero. It appears that microwave brightness temperatures are directly related to ocean surface wind speeds, regardless of fetch.