Long time series of observations of essential climate variables in thetroposphere and stratosphere are often impacted by inconsistencies ininstrumentation and ambiguities in the interpretation of the data. To reducethese problems of long-term data series, all measurements should include anestimate of their uncertainty and a description of their sources. Here wepresent an update of the uncertainties for tropospheric and stratosphericwater vapor observations using the cryogenic frost point hygrometer (CFH). Thelargest source of measurement uncertainty is the controller stability, whichis discussed here in detail. We describe a method to quantify thisuncertainty for each profile based on the measurements. We also show theimportance of a manufacturer-independent ground check, which is an essentialtool to continuously monitor the uncertainty introduced by instrumentvariability. A small bias, which has previously been indicated in lowertropospheric measurements, is described here in detail and has beenrectified. Under good conditions, the total from all sources of uncertaintyof frost point or dew point measurements using the CFH can be better than0.2 K. Systematic errors, which are most likely to impact long-term climateseries, are verified to be less than 0.1 K.The impact of the radiosonde pressure uncertainty on the mixing ratio forproperly processed radiosondes is considered small. The mixing ratiouncertainty may be as low as 2 to 3 %. The impact of the ambienttemperature uncertainty on relative humidity (RH) is generally larger thanthat of the frost point uncertainty. The relative RH uncertainty may be as lowas 2 % in the lower troposphere and 5 % in the tropical tropopauseregion.
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