M.A. Holdaway , M. Ishiguro , Scott M. Foster , R. Kawabe , K. Kohno , F.N. Owen , S.J.E. Radford , M. Saito 
April 19, 1996
Keywords: Rio Frio, Chajnantor, Chile, site testing, opacity, phase stability
We investigate the site testing data from the Rio Frio and Chajnantor sites in northern Chile over the period July 1995 through February 1996. The 225GHz opacities are about 40% higher at Rio Frio than at Chajnantor, smaller than the 65% which would be expected if the difference in opacities were due entirely to the 1000m difference in elevation (assuming a 2km scale height). The rms phase fluctuations are about 20% higher at Rio Frio than at Chajnantor. Neither site shows very much diurnal variation in opacity. Both sites show very similar diurnal variations of rms phase, phase structure function exponent, and the speed of the turbulent water vapor above the site. Most differences between the two sites in these diurnal variations can be explained in terms of the difference in elevation.
The effects of the ``Bolivian Winter'', a southern hemisphere summer weather trend in which the winds come out of the east, bringing moisture from the Amazon basin to Chile, do not affect the quality of the Chajnantor site much more than the quality of the Rio Frio site. It was initially thought that the proximity of the Chajnantor site to the source of the Bolivian Winter's moisture might adversely affect the phase stability and opacity at the Chajnantor site relative to the Rio Frio site. During the very worst phase and opacity conditions, Chajnantor does perform slightly worse than Rio Frio, but the improved performance at Chajnantor during the good times greatly outweighs this trend.
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