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ABSTRACT. We discuss properties of the atmospheric water vapor above the high Andean plateau region known as the Llano de Chajnantor, in the Atacama Desert of northern Chile. A combination of radiometric and radiosonde measurements indicates that the median column of precipitable water vapor (PWV) above the plateau at an elevation of 5000 m is approximately 1.2 mm. The exponential scale height of the water vapor density in the median Chajnantor atmosphere is 1.13 km; the median PWV is 0.5 mm above an elevation of 5750 m. Both of these values appear to be lower at night. Annual, diurnal, and other dependences of PWV and its scale height are discussed, as well as the occurrence of temperature inversion layers below the elevation of peaks surrounding the plateau. We estimate the background for infrared observations and sensitivities for broadband and highresolution spectroscopy. The results suggest that exceptional atmospheric conditions are present in the region, yielding high infrared transparency and high sensitivity for future ground-based infrared telescopes.

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The region surrounding the Llano de Chajnantor, a high-altitude plateau in the Atacama Desert in northern Chile, has caught the attention of the astronomical community for its potential as an observatory site. Combining high elevation and extremely low atmospheric water content, the Llano has been chosen as the future site of the Atacama Large Millimeter Array. We have initiated a campaign to investigate the astronomical potential of the region in the optical/infrared. Here we report on an aspect of our campaign aimed at establishing a seeing benchmark to be used as a reference for future activities in the region. After a brief description of the region and its climate, we describe the results of an astronomical seeing campaign, carried out with a Differential Image Motion Monitor that operates at 0.5 mm wavelength. The seeing at the Llano altitude of 5000 m, measured over 7 nights in 1998 May, yielded a median FWHM of 1Љ .1. However, the seeing decreased to 0Љ .7 at a modest 100 m gain above the plateau (Cerro Chico), as measured over 38 nights spread between 1998 July and 2000 October. Neither of these represents the best seeing expected in the region; the set of measurements provides a reference base for simultaneous dual runs at Cerro Chico and at other sites of interest in the region, currently underway. A comparison between simultaneous measurements at Cerro Chico and Cerro Paranal indicates that the seeing at Cerro Chico is about 12% better than that at Paranal. The percentage of optically photometric nights in the Chajnantor region is about 60%, while that of nights useful for astronomical work is near 80%.