The temperature values are taken from version 3.0.0 of the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) for the period 1880-2014 and from version 3.0.2 of ICOADS thereafter. The in situ air temperature readings recorded between one hour after sunset and one hour after sunrise have been extracted from ICOADS and the ship data have been adjusted to reduce the effects of varying thermometer heights. The data have been adjusted from their respective recording heights to each of three reference heights: 2m, 10m and 20m. The air temperature readings have been subjected to a quality-control procedure and values that fail these tests have been excluded. Duplicate values have also been excluded. Additional adjustments have been applied to the data during the Second World War to account for non-standard thermometer exposures on some ships.
The adjusted data have been aggregated into monthly mean values in each grid-cell; uncertainty estimates of these gridded values are also provided. The data have not been interpolated across missing grid boxes. In addition to the absolute temperature values, the gridded data also contain anomalies that are expressed with respect to three climatological averages (1961-90, 1971-2000 and 1981-2010).