Abstract
Ten year statistics are presented of the use of the 2.1 -m telescope of the Observatorio Astronomico Nacional at San Pedro Martir, Baja California, Mexico. Also included are some basic climatic and meteorological parameters such as temperature and relative humidity as well as the amount of cloud cover. The fraction of nights lost due to bad weather was 27.4% in the period July 1982 to June 1992. From January 1984 to December 1991,56.7% of the nights were of "photometric" quality and 80.4% were of "spectroscopic" quality. The average relative humidity near the ground was 54%. Spring and autumn are the best seasons in terms of cloudless and low humidity nights while winter is affected by the tails of North Pacific storms and mid summer is characterized by a mild monsoon season. The total cloud cover percentage in San Pedro Martir is similar to that of Cerro Tololo and La Silla, slightly better than Mauna Kea and considerably better that Mount Graham, especially in summer.
