We have been monitoring meteorological conditions since 1996 at two possible sites in northern Chile -- Pampa La Bola and Llano de Chajnantor -- for the future large millimeter and submillimeter array. The compiled meteorological data are here evaluated to compare the observing conditions at these sites. The sites at Pampa La Bola and Llano de Chajnantor are both known to be excellent for astronomical observations at millimeter and submillimeter wavelengths. There are, however, measurable differences in the meteorological conditions at these two sites. The wind was significantly weaker at Pampa La Bola than at Llano de Chajnantor during the nighttime (UT-4h = 22h--9h) of the summer (January--April) and marginally weaker during the daytime of the winter, while it was marginally stronger during the daytime of the summer and the nighttime of the winter. On the other hand, the wind flow appeared to be slightly less organized (as determined from the variation of wind direction) at Pampa La Bola than at Llano de Chajnantor, possibly because of the mountains located to the west of Pampa La Bola. The site at Pampa La Bola had a marginally larger fraction of clear days than did the site at Llano de Chajnantor. The near-surface temperature was lower at Llano de Chajnantor by 1.8 K on average, probably reflecting the 250 m height difference between the two sites. During many summer mornings, however, Pampa La Bola was colder than Llano de Chajnantor. This is likely due to a temperature inversion in the lower atmosphere at the altitude of the sites. The surface water vapor pressure was generally lower at Pampa La Bola than at Llano de Chajnantor. Since we know from other measurements that the total precipitable water above Llano de Chajnantor is generally lower than that at Pampa La Bola, the fact that the surface water vapor pressure is lower at Pampa La Bola must indicate that this quantity is dominated by local surface effects, and is not representative of the bulk atmosphere above the two sites.
View a PDF version of ALMA Memo #322.
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