Notes on meeting with Readhead on CBI experience in Chile. Wootten, with some words from Simon's and Bryan's emails... On 2000 November 6, Monday, Tony Readhead, Bob Brown, Al Wootten, Simon Radford and Bryan Butler met in Brown's office to discuss observing and operating conditions at the Chajnantor site. The Cosmic Background Imager (CBI) group has learned a lot about Chajnantor while operating the CBI this past year and what wisdom they can provide to the ALMA project so that it can benefit from that knowledge. The CBI group brought their equipment to the Chajnantor altiplano in August 1999 and began operations in November 1999. The project is funded through next austral Fall; additional funding has been sought to extend this to August 2001. The CBI is a 13-element interferometer mounted on a 6 meter platform operating in ten 1-GHz frequency bands from 26 GHz to 36 GHz. It sits in a clamshell dome about a kilometer west of the ALMA site characterization containers, Mostly, the experience on the site has been that the weather can be spectacular--the last run of 46 nights have been limited by the receiver noise, for example, with sky undetectable. This can occur a very few times a year at OVRO, for comparison,leading tony to estimate that chajnantor is 2 orders of magnitude 'more efficient' for observing than OVRO. But about a dozen nights have been problematic at Chajnantor but apparently not bad at Pampa la Bola. For a short term experiment, 10 per cent extra observing time would be a great boon. These nights can be characterized as follows: clouds form in the early evening over Chajnantor, often with snow following. CBI cannot open up. The 'far field' for that instrument is only a few 100m above the site, so they actually image the atmosphere as well as the distant Universe. Sometimes, the layer falls below the site after or about midnight. Formation of the cloud layer is seen also in the relative humidity, which rises through 90% during these events. Power is clearly seen on shorter baselines, an indication that liquid water is a component of the clouds in addition to snow. Condensation occurs on the dewar windows during these events, a further impediment to observing. Wind has not caused much time loss, nor has inaccessibility of the site. Ice formation on equipment has not caused problems either. As the team heads back to San Pedro from the CBI site, they pass through Pampa la Bola, where the clouds and snow have not occurred. Presumably, these microclimatic events occur as moist air arrives at the top of the plateau and undergoes a phase change. Tony estimaged the number of nights when it actually snowed to be around 30-40 nights of "bad snow", and 4 months of "sporadic snow" in the period from mid-December to early-June. Wind did not often pose a problem though there was a very bad wind storm in july, where there were gusts up to 100 mph, and the "mean wind" was about 40 mph over a 30 hour period. they were quite worried about the integrity of the CBI (and were lucky that they had reinforced it in Pasadena). a few things failed, but nothing catastrophic. Erasmus' predictions for wind are pretty good. Tony thinks that the wind spec for the antennas should include a time period. There is currently no time attached to the wind spec (which is 65 m/s). Peter said he would think about it. Tony thinks that the LMSA/NRO container isn't the right place to really do comparative testing between Pampa la Bola and Chajnantor. His feeling is that the wind there is bad because it is "funnelled" between Cerros Chajnantor and Toco, and so of all the places on Pampa la Bola, it is maybe the worst. The primary objective now is understanding these "micro climate" episodes, where clouds form over Chajnantor but not over Pampa la Bola. Tony estimated the CBI had lost perhaps a dozen nights this year because of this effect. Perhaps the moisture from the valley turns into water droplets and ice over the altiplano. By correlating the CBI logs with the data from the other instruments (tippers, etc.) can we recognize these episodes? Steve Myers plans to try to identify the episodes from the CBI logbooks. At other times, 'invisible clouds' cause other effects. Sometimes increased power on shorter baselines/water in virtual clouds can be seen in the CBI data and might be detectable with the ALMA equipment. Under worse conditions, the CBI receivers individually detect different sky conditions, as shown when total power levels vary incoherently across the CBI array. These events might have occurred but gone unrecognized in ALMA tipper measurements. Every so often, the tipper stares straight up and measures the variation of power on the sky. This should show the water containing pockets which cause (a) the excess of power on short CBI baselines and the (b) power level fluctuations on individual antennas. The phase measurements occur on longer baselines and might not detect the microclimatic episodes; we can look for this once the event times are identified. Unfortunately, other tippers at the site do not have a 'sit and stare' mode. Nights when the CBI registers an excess of power on the shorter baselines might be nights when 'anomalous refraction' occurs. This also correlates well with relative himidity. Tony estimated that this had occurred on only a few nights. This has been estimated by extrapolation of data from the 300m STI measurements to 12m baselines. However, a direct estimate at short scales would be of great use. Examination of data from these nights might give us some insight into the structure of the pockets causing the effect. We will try to identify these events for further investigation. Other action items: *correlate CBI evidence for liquid water in invisible clouds with 225 GHz tipper stare mode fluctuations. The 183 GHz radiometers don't tip, but stare all the time. We might also see something in those data. Simon will discuss this with Richard Hills next week. *snow, and black ice on the tarmac, are dangerous. Several accidents have occurred this year alone. The Jama road may not be cleared while drifting continues, limiting access for periods which can reach 3 days. *correlate RH meters at ALMA/EU and LMSA sites with the CBI results. *consider ice prophylactic measures on antennas--watch out for open conduits *doors on tepees should be upwind. *consider weather stations at sites planned for array centers.