Continuous measurements of the C band normalized radar cross section (NRCS) at vertical/vertical polarization as a function of wind speed and direction were carried out from a research platform in the North Sea during a period of 11 months in 1984. The NRCSs have been correlated with other environmental data routinely collected at the platform, like air-water temperature difference, water temperature, and ocean wave slope. The wind speed exponent of the C band NRCS at 45¿ incidence angle was determined as 1.50, valid in the wind speed range 2--19 m/s. Our data set shows that the C band NRCS depends strongly on the air-sea temperature difference. For example, when the air-sea temperature difference decreases from +5 ¿C to -5 ¿C, the NRCS increases by 6 dB at a wind speed of 8--9 m/s and an incidence angle of 45¿. For stable air-sea interfaces and low wind speeds the NRCS increases with increasing root-mean-square wave slope. However, no systematic dependence on water temperature was observed. ¿ American Geophysical Union 1989