Table 3-8. Meteorological Source Flags
Flag Definition
A   Data as received from NCDC, converted to SI units
B   Linearly interpolated to fill short data gaps
C   Non-linearly interpolated to fill data gaps from 6 to 47 hours in length
D   Long data gaps from 48 hours to one year filled from other years
E   Modeled or estimated, except: precipitable water, calculated from
  radiosonde data; and aerosol optical depth, calculated from direct
  normal data
F   Precipitable water, calculated from surface vapor pressure; aerosol
  optical depth, estimated from geographic correlations
?   Source does not fit any of the above. Used mostly for missing data

Notes: Flag B: For sky cover, temperature, and relative humidity, "short" is less than
        6 hours. For precipitable water, "short" is less than 60 hours. Data gaps in
        other meteorological elements were not filled.

        Long gaps in pressure data were filled with the climatological mean pressure
        for the station.