Table 3-5. Meteorological Source Flags

  Flag       Definition    
A  Data as received from NCDC, converted to SI units
B  Linearly interpolated
C  Non-linearly interpolated to fill data gaps from 6 to 47 hours in
D  Not used
E  Modeled or estimated, except: precipitable water, calculated from
  radiosonde data; dew point temperature calculated from dry bulb
  temperature and relative humidity; and relative humidity calculated  
  from dry bulb temperature and dew point temperature
F  Precipitable water, calculated from surface vapor pressure; aerosol  
  optical depth, estimated from geographic correlation
?  Source does not fit any of the above. Used mostly for missing data