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Precipitable water vapour content above the Roque de los Muchachos Observatory from GPS estimations

The requirements for current large and future extremely large telescopes as well as the quick development of IR instrumentation demand a proper characterization of precipitable water vapour (PWV) above astronomical sites. A comparison of PWV estimations from a photometer and a GPS (Global Positioning System) above the Observatorio del Roque de los Muchachos (ORM, La Palma, Canary Islands, Spain) was carried out and it was found a linear relation between both measurements. Such relationship will allow the calibration of the GPS measurements recorded at ORM for the period June 2001- December 2008. These large time series of PWV estimations from GPS were used to perform a statistical analysis of water vapour content above this astronomical site. Average annual value of night-time PWV of 4.86 mm was found. It was also found a clear seasonal behavior of the PWV above ORM, with smaller water-vapour columns during winter nights (average 3.36 mm). The largest values of PWV are reached in the summer nights (average 6.75 mm). The data indicate that a significant percentage of nights (~38%) are well suited for thermal infrared observations (with PWV < 3 mm), and 71% of nights present a "fair" or better IR observation opportunity at ORM.