## Comparison of the implementation

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.