| Summary: | We use a fifty-year record of wintertime radiosonde observations at the South Pole to estimate the precipitable water vapor column (PWV) over the entire period. Humidity data from older radiosondes is of limited reliability; however, we think an estimation of PWV is possible using temperature data because the wintertime lower troposphere is very close to saturated. From temperature data we derived PWV_SAT which is the PWV if the troposphere was saturated over the entire column. Comparisons to recent radiosonde humidity data indicate that PWV ≃ 0.88PWV_SAT. Since 1998 a CMU/NRAO 860 GHz atmospheric radiometer has been operating at the South Pole producing zenith opacity data, τo. It is expected that τo ∠PWV, and also τ_o ∠PWV_SAT, since the lower atmospheric column is near to saturation. We compare trends in τo, PWV_SAT, and PWV. PWV and PWV_SAT showed little trend in the last fifty years, 1961 to 2010, except perhaps in the last two decades, when PWVSAT was below average, followed by an increasing trend to above average. This increasing trend in the last decade was also observed in τo, except for the final two years when it appears that something changed in the instrument response. PWV_SAT is a useful metric for estimating PWV in the earlier years of wintertime South Pole radiosonde, and it is generally useful for evaluating the wintertime performance of radiosonde humidity and atmospheric opacity instrumentation. © 2012 American Geophysical Union. Received 16 March 2012; accepted 25 May 2012; published 7 July 2012. We thank Simon Radford for providing the NRAO/CMU 860 GHz zenith opacity data used in this study. We thank Steve Padin for useful discussions and for commenting on an early version of this manuscript. R.A.C. acknowledges partial support under NSF grant AST-0838261 to the Caltech Submillimeter Observatory and partial support under the CU/NIST PREP program. Any reference to a specific product or service does not constitute an endorsement by the National Institute of Standards of ...
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