  Sierra Negra, one of the highest peaks in central Mexico, is the site of the
Large Millimeter Telescope. We describe the first results of a comprehensive
analysis of the weather data measured in situ from October 2000 to February
2008 to be used as a reference for future activity in the site. We compare the
data from two different stations at the summit considering the accuracy of both
instruments. We analysed the diurnal, seasonal and annual cycles for all the
parameters. The thermal stability is remarkably good, crucial for a good
performance of the telescopes. From the solar radiation data we developed a new
method to estimate the fraction of time when the sky is clear of clouds. We
show that our measurements are consistent with a warm standard atmosphere
model. The conditions at the site are benign and stable given its altitude,
showing that Sierra Negra is a extremely good site for millimeter and high
energy observations.
