Sample file format:
  yr  mo da hhmm pos  vis  extCoef   lux  DN status
2008  7 20 1448  2   16093  0.19   0.000 1 0000
2008  7 20 1454  2   16093  0.19   0.000 1 0000
2008  7 20 1500  2   16093  0.19   0.000 1 0000

where:
   yr = year
   mo = month
   da = day of month
   hhmm = time (UTC/GMT)
   pos = instrument position on concrete slab
   vis = visibility (m)