When using netCDF files from CAMS European Air Quality forecasts from the ADS in ArcGIS, the 'time' values may not be displayed correctly.

In netCDF files which follow the CF conventions the 'time' variable  is normally encoded with units  of the form 'hours since ####', where ### is  a reference date and time. The 'time' variable values are an offset or 'step' from this reference date and time.

As the CAMS European Air Quality forecasts from the ADS in netCDF files are not currently CF compliant (the 'time' units are simply given as "hours"), then ArcGIS assumes a default reference date and time (such as 01-01-1899), and the data 'time' values are shown (incorrectly) as being offsets from this reference date and time.

The actual 'time' values  can be determined by looking at the 'time' variable attributes in the file, and the variable values (which give the 'step'). In the example below, the forecasts start from 00 UTC on 20200701, and have 'steps' of  0, 6, 24, 30 from this reference date and time.

//  float time(time) ;
        time:long_name = "FORECAST time from 20200701" ;
        time:units = "hours" ;
    float co_conc(time, level, latitude, longitude) ;
        co_conc:_FillValue = -999.f ;
        co_conc:species = "Carbon Monoxide" ;
        co_conc:units = "µg/m3" ;
        co_conc:value = "hourly values" ;
        co_conc:standard_name = "mass_concentration_of_carbon_monoxide_in_air" ;// global attributes:
        :title = "CO Air Pollutant FORECAST at the Surface" ;
        :institution = "Data produced by Meteo France" ;
        :source = "Data from ENSEMBLE model" ;
        :history = "Model ENSEMBLE FORECAST" ;
        :FORECAST = "Europe, 20200701-20200702+[0H_6H]" ;
        :summary = "ENSEMBLE model hourly FORECAST of CO concentration at the Surface from 20200701-20200702+[0H_6H] on Europe" ;
        :project = "MACC-RAQ (http://macc-raq.gmes-atmosphere.eu)" ;
data: time = 0, 6, 24, 30 ;
}

Other software may also show similar behaviour when used with CAMS European Air Quality forecasts netCDF data from the ADS.

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