The Large Millimeter Telescope Alfonso Serrano (LMT) is located in Puebla, Mexico, at an altitude of 4580 m. It is currently the largest single-dish telescope constructed to observe the Universe at wavelengths between 0.85 and 4 mm. Identifying interstellar molecules, exploring dense dark clouds, and understanding the properties of cold matter and interstellar dust in the local and distant Universe are among its main scientific goals. Since June 2013, the LMT has conducted four shared-risk early science campaigns∗ observing at 1.1 mm (AzTEC), and 3 mm (RSR) with the aid of a new meteorological and radiometer system to guide the exibility scheduling of observing time. Here we report measurements of the atmospheric opacity taken with this radiometer at 225 GHz between June 2013 and April 2016. These measurements show that the LMT site has exceptional weather conditions with opacities < 0:06 25% of its observing time during the driest months of December, January and February, excellent weather conditions with opacities < 0:1 50% of the same time, and opacities below 0.28 80% of the time during the entire dry season, making it a very convenient site for sub-millimeter/millimeter astronomy.