Sierra Negra, one of the highest peak in central Mexico, has been chosen as the site of the Large Millimeter Telescope as it combines high altitude and extremely low atmospheric water content. We have initiated a campaign to investigate the astronomical potential of the site in the optical/infrared. Here we report on one aspect of our campaign to be used as a reference for future activity in the site. After a brief description of the region we describe, for the first time, detailed analysis of weather data measured in situ from October 2000 to May 2002. The results show mostly benign climatic conditions, give the altitude of the site.