During days with strong insolation and low wind speed, there may be uneven net heating of the water layer near the surface of the ocean caused by variations in horizontal temperature at the sea surface. The heat loss from the water caused by evaporation, sensible, or longwave radiation is proportional to the sea surface temperature and is, therefore, greater from warm water compared to that from the relatively colder water. As a result, under low wind speed conditions and clear skies, the horizontal SST discontinuities, occurring at fronts, eddies, or in storm wakes, may diminish or even vanish. This phenomenon is illustrated here with some field and modeling results. The time dependence is important for the impact on remote sensing of SST, and it is found to be short enough that substantial masking of SST gradients can occur during the first six hours of the diurnal heating cycle, but the effect would continue to grow if calm and solar heating persist for several subsequent days. An integrated effect of this uneven net heating is seen in the seasonal masking of subsurface temperature gradients in the Gulf of Mexico and Florida Straits