Clear, calm ocean surfaces have very low emissivity values and appear relatively cold at lower microwave frequencies. Winds act on the ocean surface and generate foam and millimeter-sized capillary waves. Both of these mechanisms act to "roughen" the ocean surface at the lower microwave frequencies and hence, increase the emissivity. These changes are detected under rain-free conditions since rain also acts to "roughen" the ocean surface, creating a false wind signature. Brightness temperature measurements in the AMSU 31-GHz (37-GHz for SSM/I) channel are most successful in correlating the "roughening" of the ocean surface with surface wind speed. Other neighboring frequencies are used to remove contamination by clouds and water vapor.

Accuracy of the ocean wind speed product is on the order of 2-5 m/s with an effective range of between 0 and 30 m/s (108 km/hr). 108 km/hr is incidentally just below the threshold for a minimal hurricane.

Passive microwave radiometers cannot retrieve wind direction. There are however other means available for determining wind direction ranging from numerical model data to cloud motions and surface pressure patterns. Active microwave sensors, called scatterometers, can retrieve wind vectors directly. A limited number of research scatterometers, like the one on the European Remote Sensing Satellite (ERS-2), are generating limited sets of wind fields. More such instruments are planned for various POES research missions in the near future and could provide another valuable dataset to operational forecasting.