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There have been astronomical observatories on Magdalena Ridge in south-central New Mexico since the late 1960s. Magdalena Ridge is relatively flat, at an average elevation of 10,560 feet (3220 meters) with a north-south length of 3/4 of a mile. In 2000 the Magdalena Ridge Observatory began site testing for two new facilities: a 2.4-meter optical telescope and a 10-element optical interferometer. As part of that testing, meteorological instrumentation was deployed at several locations across the mountain. As a result, we have an 18 year history of regular experience with the environment, including weather and cloud cover data for much of this time period. We present trends in the basic meteorological parameters: temperature, humidity, barometric pressure, wind speeds and directions, and cloud cover. Diurnal temperatures ranges vary from 15 C° in the spring when it is largest to 10 C° in the summer months when it is smallest. Barometric pressure varies more in the spring and fall than in the summer. Annual rain fall levels vary greatly with an average of about 10 inches of rain per year. The snow amounts have traditionally been very hard to measure as the area is partly above the tree line and wind-blown snow can leave parts of the region barren while other parts have a foot or more of snow. Winds speeds are typically 10 to 20 miles per hour. Wind speeds have been measured above 100 mph (45 m/s), with wind gusts as high as 125 mph (56 m/s), though this is primarily a spring phenomenon. The wind direction is predominately out of the Southwest. Wind speeds at the 2.4-meter telescope location are frequently 2 times as high wind speeds at the optical interferometer site due to the differences in terrain to the West of the two sites. An optical allsky camera has been in operation on the Ridge from 2003 to 2012 with nightly sequences of images obtained on most nights when the winds were less than 15 m/s and the humidity below 90%. Analysis of this imagery shows that a majority of the nights would be useable for astronomical observations. We present an overview of statistics of the site and discuss how these statistics will be used for defining appropriate operational windows for the Magdalena Ridge Observatory Interferometer.
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