code: def _compute_airmass(self):
      air_mass = 1.0 / self._mus + 1 / self._muv
      air_mass[air_mass > MAXAIRMASS] = -1.0
      return air_mass
  
imports: from __future__ import annotations
  import logging
  from typing import Optional, Type, Union
  import dask.array
  import numpy
  import xarray
  from satpy.dataset.dataid import WavelengthRange
label_desc: Use image node_id 2 for calling the _VIIRSAtmosphereVariables obj's underlying member method code with example usage: obj._compute_airmass() and returns: air_mass