Air-sea observations made by the Long-Term Upper Ocean Study in the Sargasso Sea are used to determine how the diurnal response of sea surface temperature ΔTs depends upon surface heating H and wind stress S. The data set analyzed here is extensive, spanning two full summers and 361 days in total. It samples virtually the full range of possible heating conditions, and ΔTs varies from zero up to 3 ¿C. There is a clear trend of increasing ΔTs with increasing H or decreasing S (the qualitative result expected). A smooth function calculated as an average from the data itself can account for about 90% of the variance of ΔTs, showing a direct and significant dependence of ΔTs upon H and S. The analyzed data are compared to a theoretical model developed under the assumption that wind stress causes vertical mixing via shear flow instability. This model can account for about 80% of the observed variance and shows an H, S dependence consistent with the data. ¿ American Geophysical Union 1987