README
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Final vs preliminary data
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The daily surface air temperature data included in the CSV files are daily
averages calculated from 24 hourly values from 00 to 23 UTC based on ERA5
analysis fields (i.e. in which surface temperature observations are assimilated).
These are marked as 'FINAL' in the CSV file. The last day is marked as
'PRELIMINARY' because in this case the daily average is based on ERA5 hourly
analysis fields for 00 to 21 UTC and ERA5 short-term forecast fields for 22 and
23 UTC. When the hourly analyses for 22 and 23 UTC become available, the daily
average is updated to produce the final value.

The case of SST is a little different because the satellite-based SST data that
are used in ERA5 are already daily average products. They are kept constant
during the ERA5 analysis cycles. Therefore, to produce the time series included
in the CSV files, we use the SST field from ERA5 at 12 UTC and therefore don't
have to wait for the data at 22 and 23 UTC. That's why there are no preliminary
values for SST.