Determining the most plausible fit for the NH₃ doublet. The vertical profile is parameterised in terms of a degree of saturation (panel a), with relative humidities given as percentages in the key, based on the best-fitting T(p)), an upper-level cut off (panel c), with pressures between 250–860 mbar given in the key) and a deep mole fraction (panel d)), with mole fractions given in ppm in the key). Of the three parameters, the only way to increase the depth of the absorption features to match the SPIRE data (black circles) is to increase the relative humidity (panel a)). The models in c) and d) do not match the data because they assume a relative humidity of 100%. The Δχ² for each value of the relative humidity is evaluated for data between the two dotted vertical lines in panel a (i.e., avoiding artefacts in the data near 41.5 cm^-1) and displayed in panel b). The best-fit mole fraction at 660 mbar plus the 3σ uncertainty is shown in panel b).