The HadISST1 data set is a blend of historical SST and modern SST observations from ships, buoys, drifters, etc. and sea ice observations, partly from historical ship- and air-borne and partly from satellite data.
The SST data are taken from the Met Office Marine Data Bank (MDB), which from 1982 onwards also includes data received through the Global Telecommunications System (GTS). In order to enhance data coverage, monthly median SSTs for 1871-1995 from the Comprehensive Ocean-Atmosphere Data Set (COADS) (now ICOADS) were also used where there were no MDB data. The sea ice data are taken from a variety of sources including digitized sea ice charts and passive microwave retrievals.
For a detailed description of the dataset, see the web page at the Hadley Centre and the paper by Rayner et al. (see references).
The top right figure shows the time series of the global mean HadISST SST. We NOTE EXPLICITELY, that this time-series is based on a SIMPLE ARITHMETIC MEAN of the SST of the ice-free grid cells WITHOUT ANY WEIGHING with information about data density and quality - like is used to create the global SST and temperature anomalies of the HadCRUT and CRUTEM data sets.
Last update of this data set at ICDC: June 28, 2018