This unit computes 1-minute and 10-minute average visibility ranges.
The data were sampled at 1 minute intervals and averaged over 6 minutes.

Sample file format:
  yr mm da time pos alm1 alm2  vis     vis10  temp  lum
2009  3  1 0212  5 000  0000    1458    1555   7.3   6
2009  3  1 0218  5 000  0000    1615    1478   7.3   6
2009  3  1 0224  5 000  0000    2300    1629   7.3   3

where:
   yr = year
   mo = month
   da = day of month
   hhmm = time (UTC/GMT)
   pos = instrument position on concrete slab
   vis = visibility, 6-minute average (m)
   vis10 = visibility, 10-minute average (m)
   alm1 = three fields: visibility alarm 1, 2=visibility alarm 2, 
             3 = visibility alarm 3    
   alm2 = four fields: 1=hard error, 2=hard warning, 3=backscatter alarm,
             4=backscatter warning
   temp = temperature, deg C
   lum = background luminance (cd/m^2)