An array of instruments continues to monitor weather conditions at the Chajnantor site. Data are retrieved, reduced, and placed at the ALMA WWW site for inspection by casual or interested observers. There are at present a number of opacity measuring devices at Chajnantor, operating over frequencies including 183 GHz, 225 GHz, 492 GHz, 350 microns, 260 microns and 200 microns, and for some of these from different locations, over relatively rapid timescales. Both the Japanese and the Smithsonian have operated FTS devices, for various lengths of time; this gives the best measure of the atmospheric transmission over frequency. These data will be discussed at an IAU Technical Workshop in Fall 2000 in Morocco.
Two memos (176 and 322) compare conditions at the sites. Memo 176 concluded the phase stability is somewhat better on Chajnantor than on Pampa la Bola. Memo 322 found the differences in the basic meteorological data for the two sites to be small. In addition, a Japanese study by Sakamoto et al. compared the 220-225 GHz transparency data for Chajnantor and Pampa la Bola during 1999 August - December. Chajnantor consistently had better transparency.
During the first part of 2000, Y2K concerns, along with the first recorded major lightning strike at Chajnantor, have caused interruptions in the flow of data from Chajnantor, though instruments at Pampa La Bola continued recording data. Tipper data is sparse for January and February, and interferometer data sparse through May. The distributions, etc., are quite comparable to previous summer and autumn seasons.
Recently, the observers at the CBI experiment on Chajnantor have communicated their experiences with the site. Owing to cloud, snow and high winds they have been able to observe 46% of night time during the Altiplanic Winter and about 60% during the last 9 months. Snow drifts on the paved highway and on the dirt road leading to the the Chajnantor area made access difficult for a significant part of 2000.
The ASAC strongly recommends that in addition to the ongoing site characterization studies, information on precipitation and all-sky cloud coverage should be collated. Here, information from the CBI logbooks should provide important subsidiary information. Furthermore, a thorough comparison of the Pampa la Bola and Chajnantor sites should be made using as many contemporaneous data as possible.