The following are the sigma levels used by the model 
(to 4 decimal places).  The model has 7 levels above
100 mb, and 5 below 900 mb (assuming sea-level).

    sigma = pressure / surface_pressure


  0.0027
  0.0101
  0.0183
  0.0288
  0.0418
  0.0580
  0.0782
  0.1028
  0.1326
  0.1682
  0.2101
  0.2582
  0.3125
  0.3720
  0.4357
  0.5017
  0.5681
  0.6329
  0.6943
  0.7508
  0.8014
  0.8458
  0.8838
  0.9159
  0.9425
  0.9644
  0.9821
  0.9950