Characteristics of full resolution (L2P) CCI SSTs (derived from ATSR and AVHRR imagery). (a) Number of SST observations per unit area of ocean (“observation density”), per sensor over time. Each dot represents the observations of a single day. The upper panel shows data for the quality levels recommended for climate applications (QL 4 & 5), and the lower panel shows QL 3. (b) Zonal SST observation density per unit area, for all sensors combined and averaged over the time series. (c) Histogram of evaluated uncertainty in SST observations, for all sensors combined over the time series. (d) Histogram of evaluated sensitivity of SST observations, for all sensors combined over the time series.