Processing of data is described in: 

Kent, E. C., N. A. Rayner, D. I. Berry, M. Saunby, B. I. Moat, J. J. Kennedy, and D. E. Parker (2013), Global analysis of night marine air temperature and its uncertainty since 1880: The HadNMAT2 data set, J. Geophys. Res. Atmos., 118, 1281–1298, doi:10.1002/jgrd.50152.

http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50152

Due to a processing error data are missing for March 1994. Anomalies for this month have been infilled with an average of anomalies for February and April 1994, where both are present. Uncertainties for March 1994 have been conservatively estimated as the sum of uncertainties for the surrounding months.

Anomalies are relative to the climatology given in the file. It was calculated based on an earlier climatology with a different reference height in the following way. A climatology of adjustments has been calculated where there are 3 or more values in the period 1961-1990. Gaps of a single month are interpolated then gaps of a single 5-degree longitude are interpolated. Missing data towards the poles where the original climatology exists are then replaced with the most poleward value for each latitude band. The adjustment climatology is then added to the original climatology. For each 5-degree grid-box the mean difference between the climatology and the data (actuals) is then removed. Data are then set to missing if there are not at least 36 monthly values within the 30-year climatology period. The resulting climatology is slightly less extensive than the earlier climatology, as a result there are slightly fewer filled grid boxes in HadNMAT.2.0.1.0 than there were in HadNMAT.2.0.0.0.

Note that the URL given in the NetCDF file is out of date. The new URL for the hadobs web page is http://www.metoffice.gov.uk/hadobs

netcdf HadNMAT.2.0.1.0 {
dimensions:
        longitude = 72 ;
        latitude = 36 ;
        time = 1572 ;
        nv = 2 ;
variables:
        float longitude(longitude) ;
                longitude:units = "degrees_e" ;
                longitude:modulo = 360.f ;
                longitude:standard_name = "longitude" ;
                longitude:long_name = "longitude" ;
        float latitude(latitude) ;
                latitude:units = "degrees_n" ;
                latitude:standard_name = "latitude" ;
                latitude:long_name = "latitude" ;
        float time(time) ;
                time:units = "days since 1850-1-1 0:0:0" ;
                time:long_name = "time" ;
                time:bounds = "time_bnds" ;
                time:calendar = "gregorian" ;
        float time_bnds(time, nv) ;
        float night_marine_air_temperature_anomaly(time, latitude, longitude) ;
                night_marine_air_temperature_anomaly:units = "degC" ;
                night_marine_air_temperature_anomaly:long_name = "HadNMAT2 night marine air temperature anomalies from 1961-1990 mean" ;
                night_marine_air_temperature_anomaly:standard_name = "air_temperature_anomaly" ;
                night_marine_air_temperature_anomaly:ancillary_variables = "night_marine_air_temperature_climatology" ;
                night_marine_air_temperature_anomaly:missing_value = -999.f ;
                night_marine_air_temperature_anomaly:_FillValue = -999.f ;
        float night_marine_air_temperature(time, latitude, longitude) ;
                night_marine_air_temperature:units = "degC" ;
                night_marine_air_temperature:long_name = "HadNMAT2 night marine air temperature" ;
                night_marine_air_temperature:standard_name = "air_temperature" ;
                night_marine_air_temperature:ancillary_variables = "large_scale_correlated_uncertainty" ;
                night_marine_air_temperature:missing_value = -999.f ;
                night_marine_air_temperature:_FillValue = -999.f ;
        float night_marine_air_temperature_climatology(time, latitude, longitude) ;
                night_marine_air_temperature_climatology:units = "degC" ;
                night_marine_air_temperature_climatology:long_name = "HadNMAT2 night marine air temperature monthly climatology, 1961-1990" ;
                night_marine_air_temperature_climatology:missing_value = -999.f ;
                night_marine_air_temperature_climatology:_FillValue = -999.f ;
        float uncorrelated_uncertainty(time, latitude, longitude) ;
                uncorrelated_uncertainty:units = "degC" ;
                uncorrelated_uncertainty:long_name = "Uncertainty from errors unlikely to be correlated" ;
                uncorrelated_uncertainty:missing_value = -999.f ;
                uncorrelated_uncertainty:_FillValue = -999.f ;
        float large_scale_correlated_uncertainty(time, latitude, longitude) ;
                large_scale_correlated_uncertainty:units = "degC" ;
                large_scale_correlated_uncertainty:long_name = "Uncertainty from errors likely to be correlated over large scales" ;
                large_scale_correlated_uncertainty:missing_value = -999.f ;
                large_scale_correlated_uncertainty:_FillValue = -999.f ;

// global attributes:
                :Conventions = "CF-1.6" ;
                :Reference = "Kent, EC, NA Rayner, DI Berry, M Saunby, BI Moat, JJ Kennedy and DE Parker, 2013: Global analysis of night marine air temperature and its uncertainty since 1880, the HadNMAT2 Dataset, J. Geophys. Res. Atmospheres, 118, 1281-1298, doi: 10.1002/jgrd.50152" ;
                :Title = "Monthly 5 degree Night Marine Air Temperature, HadNMAT2" ;
                :source = "HadNMAT.2.0.1.0" ;
                :history = "NetCDF file generated Sat Mar 21 18:39:38 2015" ;
                :supplementary_information = "available from http://www.metoffice.gov.uk/hadobs" ;