Measurement of PWV and Saturation-point PWV

Profiles of temperature, pressure, and water vapor above the Pole have been measured at least once a day for several decades by the South Pole meteorology office, using balloon-borne radiosondes. These have typically shown atmospheric water vapor values between 50% and 100% of saturation for air at the observed temperature and pressure. The precipitable water vapor (PWV) values consistent with saturation are, however, extremely low because the air is dessicated by the frigid temperatures (annual average: -49 C, minimum temperature: -82 C). Judging by other measures of PWV such as LIDAR and mid-infrared spectroscopy, the calibration of the hygrometers used on balloon sondes was accurate until the hygrometer type was changed in 1996; measurements since then have probably been spuriously low. 

 
Balloon Measurements of PWV at Pole. (Left) Quartiles of precipitable water vapor (PWV) distribution in winter (day-of-year 100-300) from 1991 through 1998, calculated from balloon-borne hygrometer measurements. The hygrometer type was changed in 1996, and the subsequent calibration has been spurious. This new hygrometer, the AIR model 5a, has been used in measurements at other sites, in particular the peaks surrounding the Chajnantor plateau measured by Giovanelli et al. (1999,); the 1997 Pole measurements make possible a direct comparison. (Right) Quartiles of saturation-point PWV distribution in winter (day-of-year 100-300) from 1992 through 1998, calculated from balloon-borne pressure and temperature measurements. Saturation-point PWV is calculated by assuming 100% water vapor saturation for a column of air with a measured temperature and pressure profile. Since temperature and pressure sensor calibration is more reliable than hygrometer calibration, this value gives reproducible results. If hygrometer measurements from 1991-1996 can be trusted, the winter South Pole atmosphere is, on average, 90% saturated. This figure is from a paper in preparation by R. Chamberlin of the CSO.

A firm upper limit to the PWV can be set by calculating what the PWV would be if the column of air were 100% saturated with water vapor at the observed temperature and pressure, the "saturation-point PWV". Since the temperature and pressure measurements from balloon sondes are accurate, and since the atmosphere cannot be significantly supersaturated, the ``saturation-point PWV" is a reliable upper limit to the true PWV.

 
Saturation-point PWV at Pole, 1961-1998. Quartiles of saturation-point PWV distribution during the winter (day-of-year 100-300) for 1961 through 1998, calculated from balloon-borne pressure and temperature measurements. This figure shows saturation-point PWV over the entire period of continuous weather measurements at the South Pole, showing the long-term stability of the climate. Figure courtesy of R. Chamberlin.

PWV values at the Pole are small, stable and well-understood

Quartile values of the distribution of PWV with time are plotted below, where they are compared with corresponding values for Mauna Kea and for the proposed Atacama Large Millimeter Array  (ALMA) site at Chajnantor. 

Quartiles of PWV at three sites.  For Mauna Kea and Chajnantor, the PWV values were derived from the relation between PWV and measured opacity at 225 GHz. For each site, the data are separated into the best 6-month period and the remainder of the year.

Of the three sites, South Pole has by far the lowest PWV, during Austral summer as well as winter. In fact, the 75^th percentile value for the wetter half of the year at the South Pole is less than the 25^th percentile value for the drier half of the year at Mauna Kea or Chajnantor.

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Last modified: November 03, 1999