Under the CSO weather multiplexing policy, programs that require excellent weather conditions, typically 350 μm observations, are scheduled in parallel with programs that are not so demanding, typically 230 GHz observations. Each afternoon, the CSO Technical Manager, or a designated staff member, decides which program has priority. The decision is based on the optical depth measurements, other meteorological data, and weather forecasts. The nominal threshold is a 225 GHz zenith opacity of 0.06. Any questions about the priority decision should be directed to the CSO Technical Manager.

CSO safety rules require the presence of at least two observers at the telescope at any time. For weather multiplexing, therefore, each program must provide at least one observer for all scheduled half nights. (Non multiplexed programs must provide two observers.) If the priority program has only one observer, an observer from the other program must go to the telescope as well. Observing groups are encouraged to be collegial. The two observer rule will be enforced for all observing programs.