Observations of oceanic white caps for moderate to highwind speeds

A series of photographs of sea surface whitecap conditions for wind speeds of 10 to 25 m/sec was obtained and analyzed for areal coverage of white water. The results are in good agreement with semiempirical calculations based on the wind speed and the development of the wave spectrum only when the contribution of thin foam streaks oriented in the direction of the wind is neglected. Since both the actively forming whitecaps and the thin foam streaks contribute significantly to the microwave emissivity of the sea surface, it is important that the foam streaks be included in the theory but differentiated from large white caps and foam patches. A simple relationship that accounts for the foam streaks based on the rate of energy transfer, the wind speed, and the wave spectrum is proposed. By means of empirically derived constant terms for the microwave signatures of white caps and foam streaks, this theory was adapted to the prediction of the increase in brightness temperature due to foam, with reasonable results to wind speeds of 20 m/sec.