An astronomical site survey at the Barcroft Facility of the White Mountain Research Station

We present a distillation of weather and sky condition data collected from September 2001 to November 2004 at the University of California White Mountain Research Station, Barcroft Facility. Our conclusion is that Barcroft is an excellent site for microwave observation because of a cold microwave zenith temperature, low precipitable water, and a high percentage of clear days. The solar intensity was above 80% of the theoretical maximum 66% of the time. About 71% of the daytime, the cloud cover was acceptable for observing. Median precipitable water vapor was estimated to be 1.75 mm. We measure a median opacity at 225 GHz of 0.11, which corresponds to a transmission of 89.6%. Zenith sky temperatures were determined to be 9.0 ± 0.2 K and 10.0 ± 0.6 K in Q-band (38-46 GHz) and W-band (81-98 GHz), respectively. We also demonstrate a correlation between measurements of precipitable water vapor from a weatherstation and a 225 GHz radiometer.