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The Thirty Meter Telescope (TMT) and the European Extremely Large Telescope(E-ELT) site testing teams have recently finalized their site testing studies.Since atmospheric water vapor is the dominant source of absorption andincreased thermal background in the infrared, both projects includedprecipitable water vapor (PWV) measurements in their corresponding site testingcampaigns. TMT planned to monitor PWV at the sites of interest by means ofusing infrared radiometers. Technical failures and calibration issues preventedthem from having a sufficiently long PWV time-series to characterize the sitesusing this method. Therefore, for the sites in Chile TMT used surface watervapor density measurements, which taken together with an assumed water vaporscale height, allowed for the estimation of PWV. On the other hand, the E-ELTteam conducted dedicated PWV measurement campaigns at two of their observatorysites using radiosonde soundings to validate historical time-series of PWVreconstructed by way of a spectroscopic analysis of astronomical standardsources observed at the La Silla and the Paranal sites. The E-ELT alsoestimated the median PWV for the Armazones site from extrapolation of theirParanal statistics accounting for the difference in elevation between the twoplaces; and also from archival analysis of radiosonde data available from thecity of Antofagasta by integration of the humidity profile starting from 3000 maltitude. In the case of the Armazones site, the published median of PWV byboth groups differ by about 1 mm with the E-ELT values being drier than thoseestimated by the TMT group. This work looks at some of the reasons that couldexplain this difference, among them the water vapor scale height, thehorizontal variability of the water vapor field, and an unaccounted correctiondue to a dry bias known to affect the radiosondes relative humidity sensors.

Precipitable Water Vapor: Considerations on the water vapor scale  height, dry bias of the radiosonde humidity sensors, and spatial and temporal  variability of the humidity field