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There are at least 2 main differences between the implementations of ATM
1985 and ATM 2009.
 ATM 1985 gives sensible (i.e. continuous) results even for negative
amount of water vapor while ATM 2009 displays a discontinuity for an
amount of water vapor equal to zero and then constant values for negative
amount of water vapor (See Fig. 1). The ATM 2009
behavior comes from the fact that the atmospheric profile is first
computed for an indicative humidity value and then scaled to the ture
amount of water vapor. The current version of ATM 2009 just default to
non continuous values for zero or negative amount of water vapor.
 For
, ATM 1985 gives just the emission of the atmosphere while
ATM 2008 gives the sum of the atmosphere plus the cosmic microwave
background emissions. The impact on the calibration has to do with the
comparison of Eqs. 16 and 17. A numeric
comparison should be done.
Figure 1:
Opacities,
and
as a function of precipitable
water vapor for two different frequencies: 115.3 GHz (left) and
230.5 GHz (right). ATM 2009 results are shown in red while ATM 1985 are
shown in black.

Next: Comparison at constant amount
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20140701