def METHOD_NAME(P, PB, T, flag=1):
  # calculate Mixing Ratio
  #  qs (kg kg-1)
  #  p  (Pa)
  #  T  (K)
  Pair      = P  +  PB
  epsi   = 0.62185
  es     = wrf_es(T, flag)
  qs     = 0.62185 * es / (Pair - es)
  return qs