Sea surface temperature (SST) measurements from both spaceborne and in situ sensors are analyzed to determine likely temperature differences that can arise between two measurements separated in space and time. This has implications both for satellite data validation, where an in situ measurement is compared with a remotely sensed one, and for the use of SST measurements in numerical upper ocean models, where an SST value is ascribed to or compared with one from a nearby grid point at the closest time step. It is shown that spatial separations of about 10 km and time intervals of about 2 hours can introduce rms differences of 0.2 K into the error budget of a satellite validation data set, this being an upper limit for the meaningful validation of current infrared radiometers. The length and the time scales at which the autocorrelation functions decay to 0.5 is used as acriterion for the meaningful use of SST data with ocean models, and these are found to be about 10--20 km and about 6 hours. This time constraint is not very severe, but the spatial constraint poses a significant sampling problem. On these length scales there appears to be no spatial anisotropy with respect to the dominant surface current flow direction. ¿ American Geophysical Union 1991