ICOADS Web information page (Tuesday, 07-Jan-2014 23:59:29 UTC):

Figure 3a. Numbers of observations (upper panel) and 2° boxes (lower panel) of SST per month (1820-1859) used for the enhanced statistics. The red curves show data before correction of the ZQ problem, and the blue curves show data after the correction. Red was plotted on top of blue, thus where red is not visible this indicates that data for a given month were completely eliminated by the ZQ problem prior to the correction. 2° boxes containing one or more observations were counted; 2000 boxes represents about 17% of the maximum possible (i.e., classified as ocean or coastal) global 2° boxes.

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