Context. Interest in the use of the Chajnantor area for millimeter and
submillimeter astronomy is increasing because of its excellent atmospheric
conditions. Knowing the general site annual variability in precipitable water
vapor (PWV) can contribute to the planning of new observatories in the area.
Aims. We seek to create a 20-year atmospheric database (1997 - 2017) for the
Chajnantor area in northern Chile using a single common physical unit, PWV.We
plan to extract weather relations between the Chajnantor Plateau and the summit
of Cerro Chajnantor to evaluate potential sensitivity improvements for
telescopes fielded in the higher site. We aim to validate the use of
submillimeter tippers to be used at other sites and use the PWV database to
detect a potential signature for local climate change over 20 years. Methods.
We revised our method to convert from submillimeter tipper opacity to PWV. We
now include the ground temperature as an input parameter to the conversion
scheme and, therefore, achieve a higher conversion accuracy. Results.We found a
decrease in the measured PWV at the summit of Cerro Chajnantor with respect to
the plateau of 28%. In addition, we found a PWV difference of 1:9% with only 27
m of altitude difference between two sites in the Chajnantor Plateau: the
Atacama Pathfinder Experiment (APEX) and the Cosmic Background Imager (CBI)
near the Atacama Large Millimeter Array (ALMA) center. This difference is
possibly due to local topographic conditions that favor the discrepancy in PWV.
The scale height for the plateau was extracted from the measurements of the
plateau and the Cerro Chajnantor summit, giving a value of 1537 m. Considering
the results obtained in this work from the long-term study, we do not see
evidence of PWV trends in the 20-year period of the analysis that would suggest
climate change in such a timescale.Comment: 10 pages, 14 figures, 4 table