Additional Atmospheric Opacity Measurements at Millimeter Wavelengths from San Pedro Mártir Sierra in Baja California

Measurements of the atmospheric zenith opacity at a wavelength of 1.4 mm (210 GHz) carried out at the Observatorio Astron omico Nacional in the Sierra San Pedro M artir, Baja California, M exico are presented. The data cover 251 days in 1999. Measurements were made on a continuous basis every eight minutes using a heterodyne radiometer. For this period, the total mean zenith opacity was 0.13 nepers; with mean values of total opacity for night time and day time of 0.14 and 0.12 nepers. The data presented here supplement those covering 210 days in 1992 (Hiriart et al. 1997). That year, a total mean sky opacity of 0.24 nepers at a frequency of 215 GHz was found. The present results conrm that measurements in 1992 were aected by El Ni~ no activity. By comparing the results of 1999 with similar measurements made at Kitt Peak and Mount Graham observatories in the same year, it is found that the North American monsoon that increases the sky opacity at millimeter wavelength lasted two months less in Baja than in southwestern USA.