ATMOSPHERIC OPACITY AT 215-GHZ OVER SAN-PEDRO-MARTIR-SIERRA IN BAJA-CALIFORNIA

We present radiometric measurements of the zenith optical depth at 215 GHz (1.4 mm) over the Observatorio Astronomico Nacional in the Sierra San Pedro Martir, Baja California, Mexico. In order to perform the me asurements in a continuous way, we have developed a computerized radio meter that allows unattended operation for long periods of time. Obser vations were carried out during 210 days in 1992. The median values of day-time and night-time zenith sky optical depth of 0.24 and 0.20 res pectively were found; the total median sky optical depth at this frequ ency was 0.22. To compare with other observatories, we have converted the sky optical depth to the Precipitable Water Vapor; in that case th e results are similar to those reported for Mauna Kea, Hawaii, and Pic o Veleta, Spain, thus placing San Pedro Martir as a promising site for astronomical observations from the infrared through the millimetric r egion.